[PATCH] mm, x86: pkeys: Introduce PKEY_ALLOC_SIGNALINHERIT and change signal semantics

This patch is based on the previous discussion (pkeys: Support setting 
access rights for signal handlers):

   https://marc.info/?t=151285426000001

It aligns the signal semantics of the x86 implementation with the 
upcoming POWER implementation, and defines a new flag, so that 
applications can detect which semantics the kernel uses.

A change in this area is needed to make memory protection keys usable 
for protecting the GOT in the dynamic linker.

(Feel free to replace the trigraphs in the commit message before 
committing, or to remove the program altogether.)

Thanks,
Florian