push yarrow locking down into the yarrow code to reduce contention

push yarrow locking down into the yarrow code to reduce contention

This seems to speed up creating a krb5_context a little bit, when it happens
a lot in multiple threads.

* prng.c (yarrow_lock): Rename to krb5int_yarrow_lock via macro, and change to
be non-static.
(krb5int_prng_init): Call do_yarrow_init here.
(krb5_c_random_add_entropy): Don't call it here.  Don't lock the mutex, either.
(krb5_c_random_make_octets): Don't lock the mutex.
* yarrow/ylock.h: Include k5-thread.h.
(krb5int_yarrow_lock): Declare.
(LOCK, UNLOCK): Define as macros using the k5_mutex_ macros.

ticket: new

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@16846 dc483132-0cff-0310-8789-dd5450dbe970