lib: streamline frr_pthreads, add default loop

lib: streamline frr_pthreads, add default loop

Some work on FRR's pthread wrapper.

* Provide a built-in way to synchronize thread startup
* Make utility functions take frr_pthread * instead of its integer ID
* Pass frr_pthread * as pthread start function argument
* Correct some comment styling
* Rename some variables to match naming conventions in the file
* Change parameter ordering in stop function prototype to follow the
  convention in the other functions
* Default new frr_pthreads to using a vanilla event loop

For the last point, the original goal when designing the implementation
of pthreads into FRR was to be able to use the thread.c event based
system inside pthreads. This code essentially encapuslates all the
thread.c functionality into an easy to use pthread out of the box.
Creating a new frr_pthread with a null attributes field will cause the
created frr_pthread to run a thread.c event loop. The upshot of this is
that it is now possible to safely run existing functions in a pthread in
roughly 3 lines of code. It also serves as an example / starting point
for others.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>