cleanups: better binary packet gen, size_t fixes and PACKET_* removal

I'll introduce a new internal function set named

 _libssh2_store_u32
 _libssh2_store_u64
 _libssh2_store_str

That can be used all through the library to build binary outgoing
packets.  Using these instead of the current approach removes
hundreds of lines from the library while at the same time greatly
enhances readability. I've not yet fully converted everything to
use these functions.

I've converted LOTS of 'unsigned long' to 'size_t' where
data/string lengths are dealt with internally. This is The Right
Thing and it will help us make the transition to our
size_t-polished API later on as well.

I'm removing the PACKET_* error codes. They were originally
introduced as a set of separate error codes from the transport
layer, but having its own set of errors turned out to be very
awkward and they were then converted into a set of #defines that
simply maps them to the global libssh2 error codes instead. Now,
I'l take the next logical step and simply replace the PACKET_*
defines with the actual LIBSSH2_ERROR_* defines. It will increase
readability and decrease confusion.

I also separated packet stuff into its own packet.h header file.

src/misc.c

@@ -145,7 +145,7 @@ _libssh2_ntohu64(const unsigned char *buf)
 /* _libssh2_htonu32
  */
 void
-_libssh2_htonu32(unsigned char *buf, unsigned int value)
+_libssh2_htonu32(unsigned char *buf, uint32_t value)
 {
     buf[0] = (value >> 24) & 0xFF;
     buf[1] = (value >> 16) & 0xFF;
@@ -153,6 +153,25 @@ _libssh2_htonu32(unsigned char *buf, unsigned int value)
     buf[3] = value & 0xFF;
 }
 
+/* _libssh2_store_u32
+ */
+void _libssh2_store_u32(unsigned char **buf, uint32_t value)
+{
+    _libssh2_htonu32(*buf, value);
+    *buf += sizeof(uint32_t);
+}
+
+/* _libssh2_store_str
+ */
+void _libssh2_store_str(unsigned char **buf, const char *str, size_t len)
+{
+    _libssh2_store_u32(buf, (uint32_t)len);
+    if(len) {
+        memcpy(*buf, str, len);
+        *buf += len;
+    }
+}
+
 /* Base64 Conversion */
 
 static const char base64_table[] =