Allows a process to request that stack gap was not applied to its stacks.
This is actually of limited usefulness since the gap where the stack grows is still maintained, it just allows to completely exhaust the gap on per-process basis instead of setting global security.bsd.stack_guard_pages.
[I will write the man pages updates after the code is agreed upon]
The placement of blank lines around PROC_LOCK_ASSERT() is different between this function and stackgap_status() for no obvious reason.