Use the previous stack entry protection and max protection to correctly

Use the previous stack entry protection and max protection to correctly
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