Add three new static mutexes for use by the application. This is a partial

import of changes from the threads branch.

FossilOrigin-Name: 3aad01960f92c5e77dba64ac1a6c6b063378fb97

src/mutex.c

@@ -81,7 +81,7 @@ int sqlite3MutexEnd(void){
 */
 sqlite3_mutex *sqlite3_mutex_alloc(int id){
 #ifndef SQLITE_OMIT_AUTOINIT
-  if( sqlite3_initialize() ) return 0;
+  if( id<=SQLITE_MUTEX_RECURSIVE && sqlite3_initialize() ) return 0;
 #endif
   return sqlite3GlobalConfig.mutex.xMutexAlloc(id);
 }