lib: Cleanup thread name setting to happen at start

lib: Cleanup thread name setting to happen at start

When we start a thread we always call fpt_run and since
the last commit we know os_name is filled with something,
therefore we can just set the name on startup.

This creates this output now for zebra:

sharpd@donna ~/frr2> ps -L -p 25643
  PID   LWP TTY          TIME CMD
25643 25643 ?        00:00:00 zebra
25643 25644 ?        00:00:00 Zebra dplane
25643 25684 ?        00:00:00 zebra_apic
sharpd@donna ~/frr2>

I removed the abstraction to frr_pthread_set_name because
it was snprintf'ing into the same buffer which was the
real bug here( the first character of os_name became null).
In the next commit I'll remove that api because
it is unneeded and was a horrible hack to get
this to work for the one place it was wanted.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>