Replace AcceptMutex, LockFile, RewriteLock, SSLMutex, SSLStaplingMutex,

Replace AcceptMutex, LockFile, RewriteLock, SSLMutex, SSLStaplingMutex,
and WatchdogMutexPath with a single Mutex directive.  Add APIs to
simplify setup and user customization of APR proc and global mutexes.
(See util_mutex.h.)  Build-time setting DEFAULT_LOCKFILE is no longer
respected; set DEFAULT_REL_RUNTIMEDIR instead.

Some existing modules, such as mod_ldap and mod_auth_digest gain
configurability for their mutexes.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@883540 13f79535-47bb-0310-9956-ffa450edef68