lib: add pthread manager

lib: add pthread manager

Adds infrastructure for keeping track of pthreads.

The general idea is to maintain a daemon-wide table of all pthreads,
running or not. A pthread is associated with its own thread master that
can be used with existing thread.c code, which provides user-space
timers, an event loop, non-blocking I/O callbacks and other facilities.

Each frr_pthread has a unique identifier that can be used to fetch it
from the table. This is to allow naming threads using a macro, for
example:

 #define WRITE_THREAD 0
 #define READ_THREAD  1
 #define WORK_THREAD  2

The idea here is to be relatively flexible with regard to how daemons
manage their collection of pthreads; the implementation could get away
with just some #define'd constants, or keep a dynamically allocated data
structure that provides organization, searching, prioritizing, etc.

Overall this interface should provide a way to maintain the familiar
thread.c userspace threading model while progressively introducing
pthreads.

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