Kernel: Implement some basic stack pointer validation

VM regions can now be marked as stack regions, which is then validated
on syscall, and on page fault.

If a thread is caught with its stack pointer pointing into anything
that's *not* a Region with its stack bit set, we'll crash the whole
process with SIGSTKFLT.

Userspace must now allocate custom stacks by using mmap() with the new
MAP_STACK flag. This mechanism was first introduced in OpenBSD, and now
we have it too, yay! :^)

Kernel/UnixTypes.h

@@ -26,6 +26,7 @@
 #define MAP_FIXED 0x10
 #define MAP_ANONYMOUS 0x20
 #define MAP_ANON MAP_ANONYMOUS
+#define MAP_STACK 0x40
 
 #define PROT_READ 0x1
 #define PROT_WRITE 0x2