MFC r320498

MFC r320498
  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.