Fix fork(2)-related deadlocks.

Add a library constructor for jemalloc that initializes the allocator.
This fixes a race that could occur if threads were created by the main
thread prior to any memory allocation, followed by fork(2), and then
memory allocation in the child process.

Fix the prefork/postfork functions to acquire/release the ctl, prof, and
rtree mutexes.  This fixes various fork() child process deadlocks, but
one possible deadlock remains (intentionally) unaddressed: prof
backtracing can acquire runtime library mutexes, so deadlock is still
possible if heap profiling is enabled during fork().  This deadlock is
known to be a real issue in at least the case of libgcc-based
backtracing.

Reported by tfengjun.

include/jemalloc/internal/prof.h

@@ -223,6 +223,9 @@ void	prof_tdata_cleanup(void *arg);
 void	prof_boot0(void);
 void	prof_boot1(void);
 bool	prof_boot2(void);
+void	prof_prefork(void);
+void	prof_postfork_parent(void);
+void	prof_postfork_child(void);
 
 #endif /* JEMALLOC_H_EXTERNS */
 /******************************************************************************/