REORG: include: split hathreads into haproxy/thread.h and haproxy/thread-t.h

REORG: include: split hathreads into haproxy/thread.h and haproxy/thread-t.h

This splits the hathreads.h file into types+macros and functions. Given
that most users of this file used to include it only to get the definition
of THREAD_LOCAL and MAXTHREADS, the bare minimum was placed into thread-t.h
(i.e. types and macros).

All the thread management was left to haproxy/thread.h. It's worth noting
the drop of the trailing "s" in the name, to remove the permanent confusion
that arises between this one and the system implementation (no "s") and the
makefile's option (no "s").

For consistency, src/hathreads.c was also renamed thread.c.

A number of files were updated to only include thread-t which is the one
they really needed.

Some future improvements are possible like replacing empty inlined
functions with macros for the thread-less case, as building at -O0 disables
inlining and causes these ones to be emitted. But this really is cosmetic.