There is currently no clean and efficient way to apply mprotect to all of a
program's stack, i.e. to the moving edge of a GROWSDOWN or GROWSUP mapping.
Some processes want to change these protections, particularly to set or
clear the PROT_EXEC bits on stack space.  As it is, an mprotect done to
cover the precise edge page of the mapping will have the desired effect of
changing the protection for existing pages and having that new protection
carried over to new pages grown later.  But there is no very reasonable way
of ascertaining where the edge of the mapping is if it might have grown in
the past beyond the usage at the moment.  An mprotect call that doesn't
cover the edge page splits the mapping and doesn't do what we need.

This patch adds flags that can be OR'd into the protection bits in an
mprotect system call.  PROT_GROWSDOWN means the memory lies in a GROWSDOWN
mapping and the start address of the region to be changed should be
extended down to the current low page of that mapping.  Similarly,
PROT_GROWSUP means the pages lie in a GROWSUP mapping and the length of the
region to be changed should be extended up to include its highest page.
These flags also explicitly request the (already implicit) behavior that
the protection change applied to the lowest/highest page of a growing
mapping is passed on to new pages grown later.  There are no other changes
to the mprotect behavior; in particular, the boundary in the non-growing
direction (the end address computed from the start+len arguments in the
GROWSDOWN case, and the start argument in the GROWSUP case) is as given by
the arguments to the system call.  This is desireable in the use of this
call by a process on its stack, so it can change the protections of the
growing mapping used for program stack distinctly from the protections on
the arguments, environment, and AT_* data from exec.