Problem: regression with "select" on *nix (#2940)

* Problem: build failure with select as polling mechanism

Solution: cast mailbox_handle argument to (poller_t::handle_t) like in
the reaper thread class.

* Problem: build failure due to INT_MAX use without include

Solution: include limits and climits in src/select.cpp where INT_MAX is
used

* Problem: build failure due to unused variable in select.cpp

Solution: move the declaration of int rc inside the ifdef block where
it is actually used

* Problem: reference to wrong variable in select.cpp breaks build

Solution: fix it

* Problem: family_entry_t constructor has no body, build fails on *nix

Solution: add empty inline function in the struct

* Problem: no test coverage for poll and select

Solution: add Travis jobs for them on Linux

* Problem: Travis jobs cannot run in container infra

Solution: set sudo: false as it is not required anymore

src/io_thread.cpp

@@ -38,7 +38,7 @@
 
 zmq::io_thread_t::io_thread_t (ctx_t *ctx_, uint32_t tid_) :
     object_t (ctx_, tid_),
-    mailbox_handle (NULL)
+    mailbox_handle ((poller_t::handle_t) NULL)
 {
     poller = new (std::nothrow) poller_t (*ctx_);
     alloc_assert (poller);