Clear the MAP_WIREFUTURE flag on the vm map in exec_new_vmspace() when it
recycles the current vm space.  Otherwise, an mlockall(MCL_FUTURE) could
still be in effect on the process after an execve(2), which violates the
specification for mlockall(2).

It's pointless for vm_map_stack() to check the MEMLOCK limit.  It will
never be asked to wire the stack.  Moreover, it doesn't even implement
wiring of the stack.