propagate the stack execution permissions when stack is grown down.
First, curproc->p_sysent->sv_stackprot specifies maximum allowed stack
protection for current ABI, so the new stack entry was typically marked
executable always. Second, for non-main stack MAP_STACK mapping,
the PROT_ flags should be used which were specified at the mmap(2) call
time, and not sv_stackprot.
MFC after: 1 week