call stat less often on krb5.conf

call stat less often on krb5.conf

Changes suggested by lxs to reduce stat frequency to once per second.
In parallel loops creating and destroying krb5 contexts on Mac OS X, this
seems to improve performance by 10%, though it's hard to be sure because
the times are variable.

* prof_int.h (STAT_ONCE_PER_SECOND): Define.
(struct _prf_data_t) [STAT_ONCE_PER_SECOND]: New field LAST_STAT.
* prof_file.c (scan_shared_trees_locked, scan_shared_trees_unlocked): Redefine
to do nothing for now.
(profile_update_file_data) [STAT_ONCE_PER_SECOND]: If the current time is the
same time as the last stat of the file, just return; otherwise, save away the
current time.

ticket: new
status: open

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@16847 dc483132-0cff-0310-8789-dd5450dbe970