Introduce extended set/get methods for ZMQ contexts (#3642)

* Introduce DRAFT zmq_ctx_set_ext() and zmq_ctx_get_ext() methods. Change
ZMQ_THREAD_NAME_PREFIX to allow for non-numeric thread name prefixes.

doc/zmq_ctx_get_ext.txt

@@ -0,0 +1,82 @@
+zmq_ctx_get_ext(3)
+==================
+
+
+NAME
+----
+
+zmq_ctx_get_ext - get extended context options
+
+
+SYNOPSIS
+--------
+*int zmq_ctx_get_ext (void '*context', int 'option_name', void '*option_value', size_t '*option_len');*
+
+
+DESCRIPTION
+-----------
+The _zmq_ctx_get()_ function shall retrieve the value for the option
+specified by the 'option_name' argument and store it in the buffer pointed to
+by the 'option_value' argument. 
+The 'option_len' argument is the size in bytes of the buffer pointed
+to by 'option_value'; upon successful completion _zmq_ctx_get_ext()_ shall
+modify the 'option_len' argument to indicate the actual size of the option
+value stored in the buffer.
+
+The _zmq_ctx_get_ext()_ function accepts all the option names accepted by
+_zmq_ctx_get()_.
+Options that make most sense to retrieve using _zmq_ctx_get_ext()_ instead of
+_zmq_ctx_get()_ are:
+
+ZMQ_THREAD_NAME_PREFIX: Get name prefix for I/O threads
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The 'ZMQ_THREAD_NAME_PREFIX' argument gets the string prefix of each thread
+created for the internal context's thread pool.
+
+[horizontal]
+Option value type:: character string
+Option value unit:: N/A
+Default value:: empty string
+
+
+RETURN VALUE
+------------
+The _zmq_ctx_get_ext()_ function returns a value of 0 or greater if successful.
+Otherwise it returns `-1` and sets 'errno' to one of the values defined
+below.
+
+
+ERRORS
+------
+*EINVAL*::
+The requested option _option_name_ is unknown.
+
+
+EXAMPLE
+-------
+.Setting a prefix on internal ZMQ threda names:
+----
+void *context = zmq_ctx_new ();
+const char prefix[] = "MyApp";
+size_t prefixLen = sizeof(prefix);
+zmq_ctx_set (context, ZMQ_THREAD_NAME_PREFIX, &prefix, &prefixLen);
+
+char buff[256];
+size_t buffLen = sizeof(buff);
+int rc = zmq_ctx_get (context, ZMQ_THREAD_NAME_PREFIX, &buff, &buffLen);
+assert (rc == 0);
+assert (buffLen == prefixLen);
+
+----
+
+
+SEE ALSO
+--------
+linkzmq:zmq_ctx_get[3]
+linkzmq:zmq[7]
+
+
+AUTHORS
+-------
+This page was written by the 0MQ community. To make a change please
+read the 0MQ Contribution Policy at <http://www.zeromq.org/docs:contributing>.

doc/zmq_ctx_set_ext.txt

@@ -0,0 +1,86 @@
+zmq_ctx_set_ext(3)
+==================
+
+
+NAME
+----
+
+zmq_ctx_set_ext - set extended context options
+
+
+SYNOPSIS
+--------
+*int zmq_ctx_set_ext (void '*context', int 'option_name', const void '*option_value', size_t 'option_len');*
+
+
+DESCRIPTION
+-----------
+
+The _zmq_ctx_set_ext()_ function shall set the option specified by the
+'option_name' argument to the value pointed to by the 'option_value' argument
+for the 0MQ context pointed to by the 'context' argument. The 'option_len'
+argument is the size of the option value in bytes. For options taking a value of
+type "character string", the provided byte data should either contain no zero
+bytes, or end in a single zero byte (terminating ASCII NUL character).
+
+The _zmq_ctx_set_ext()_ function accepts all the option names accepted by
+_zmq_ctx_set()_.
+Options that make most sense to set using _zmq_ctx_set_ext()_ instead of
+_zmq_ctx_set()_ are the following options:
+
+ZMQ_THREAD_NAME_PREFIX: Set name prefix for I/O threads
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The 'ZMQ_THREAD_NAME_PREFIX' argument sets a string prefix to each thread
+created for the internal context's thread pool. This option is only supported on Linux.
+This option is useful to help  debugging done via "top -H" or "gdb"; in case
+multiple processes on the system are using ZeroMQ it is useful to provide through
+this context option an application-specific prefix to distinguish ZeroMQ background
+threads that belong to different processes.
+This option only applies before creating any sockets on the context.
+
+[horizontal]
+Option value type:: character string
+Option value unit:: N/A
+Default value:: empty string
+
+
+RETURN VALUE
+------------
+The _zmq_ctx_set_ext()_ function returns zero if successful. Otherwise it
+returns `-1` and sets 'errno' to one of the values defined below.
+
+
+ERRORS
+------
+*EINVAL*::
+The requested option _option_name_ is unknown.
+
+
+EXAMPLE
+-------
+.Setting a prefix on internal ZMQ threda names:
+----
+void *context = zmq_ctx_new ();
+const char prefix[] = "MyApp";
+size_t prefixLen = sizeof(prefix);
+zmq_ctx_set (context, ZMQ_THREAD_NAME_PREFIX, &prefix, &prefixLen);
+
+char buff[256];
+size_t buffLen = sizeof(buff);
+int rc = zmq_ctx_get (context, ZMQ_THREAD_NAME_PREFIX, &buff, &buffLen);
+assert (rc == 0);
+assert (buffLen == prefixLen);
+
+----
+
+
+SEE ALSO
+--------
+linkzmq:zmq_ctx_set[3]
+linkzmq:zmq[7]
+
+
+AUTHORS
+-------
+This page was written by the 0MQ community. To make a change please
+read the 0MQ Contribution Policy at <http://www.zeromq.org/docs:contributing>.