[core] Detect pthread_getname* availability

[maxsharabayko]

1. Extracted pthread_getname_np() related changes from PR Fix Build for MacOS, FreeBSD, GCC earlier than 4.7, and Clang earlier… #2104 (by @jlsantiago0).

   • Detect availability of pthread_getname_np() and pthread_setname_np() functions in CMake.
   • Use the pthread_getname_np() and pthread_setname_np() functions if available in ThreadName class.

2. Additional minor change in the second commit: put ThreadName class inside srt namespace.

Original Description by Jose

This fixes MacOS builds for MacOS <10.6 as pthread_getname_np() and pthread_setname_np() was introduced in MacOS 10.6 and is not available in MacOS 10.5 and earlier.

The rework of the pthread_getname_np() and pthread_setname_np() allows other operating systems that support the API to use it. For instance, these functions are available in many of the BSD variants, but maybe in a different header file or may have a slightly different name. I also fixed the handling of the different function call parameters of the Apple and non-Apple implementations of pthread_setname_np(). Apple takes one parameter and most implementations take 2. This was tested with FreeBSD-11.3 and OpenBSD-6.4.

TODO

• Put ThreadName class inside srt namespace.