*: Rename thread.[ch] to event.[ch]

*: Rename thread.[ch] to event.[ch]

This is a first in a series of commits, whose goal is to rename
the thread system in FRR to an event system.  There is a continual
problem where people are confusing `struct thread` with a true
pthread.  In reality, our entire thread.c is an event system.

In this commit rename the thread.[ch] files to event.[ch].

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