Protect fork() and clone() behind a rwlock.

rlct_clone will acquire a read lock, whereas fork will acquire a write
lock. The write lock is necessary because the fork will clone the file
table, which would result in other threads' fork/clone file descriptors
not being closed. If an address space switch fd never gets closed, fork
child processes and new threads, may never switch address spaces before
they are started, which has resulted in hard-to-debug RIP=0 instr fetch
page faults.