* k5-thread.h: Restructured mutex code.

* k5-thread.h: Restructured mutex code.
(k5_debug_loc): New type, may contain file/line info if DEBUG_THREADS_LOC is
defined.
(k5_os_nothread_*): Dummy implementation of mutex lock for a single-threded
process.  Uses a flag and assert() if DEBUG_THREADS is defined, does nothing
interesting otherwise.
(k5_os_mutex*, k5_once*): General implementations, with dummy or POSIX or
POSIX-if-loaded-otherwise-dummy variants.
(k5_mutex_*): Combine OS-specific mutex implementation with optional file/line
tracking, and provide a place to instrument for other debugging or performance
data.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@16539 dc483132-0cff-0310-8789-dd5450dbe970