exec: reserve full virtual stack space at execve()

grows the main process stack at execve() time so that it can reach it's
maximum allowed size. No RAM is allocated until the stack actually uses this
space.
This prevents problems observed around address space randomization where
large heaps grew into and collided with the main process' stack. This
prevented the main stack from growing later.  Early workarounds had the
particular application allocating its stack to be large before going off
to execute the work that grew the heap; that saved the stack's virtual
space but also allocated RAM ahead of the stack needing it.

Change-Id: I131a1afb6e50ddfa580894562c05de653d2e30e7
Signed-off-by: Ray Essick <are002@motorola.com>
Reviewed-on: http://gerrit.mot.com/718024


SLTApproved: Slta Waiver <sltawvr@motorola.com>
Tested-by: Jira Key <jirakey@motorola.com>
Reviewed-by: Christopher Fries <cfries@motorola.com>
Submit-Approved: Jira Key <jirakey@motorola.com>