Unlock the malloc() locks in the child process after fork(). This gives

Unlock the malloc() locks in the child process after fork(). This gives
us working malloc in the fork child of the multithreaded process.

Although POSIX requires that only async-signal safe functions shall be
operable after fork in multithreaded process, not having malloc lower
the quality of our implementation.

Tested by:	rink
Discussed with:	kan, davidxu
Reviewed by:	kan
MFC after:	1 month