disable sbrk for all values of increment except 0

use of sbrk is never safe; it conflicts with malloc, and malloc may be
used internally by the implementation basically anywhere. prior to
this change, applications attempting to use sbrk to do their own heap
management simply caused untrackable memory corruption; now, they will
fail with ENOMEM allowing the errors to be fixed.

sbrk(0) is still permitted as a way to get the current brk; some
misguided applications use this as a measurement of their memory
usage or for other related purposes, and such usage is harmless.

eventually sbrk may be re-added if/when malloc is changed to avoid
using the brk by using mmap for all allocations.
disable the brk function

the reasons are the same as for sbrk. unlike sbrk, there is no safe
usage because brk does not return any useful information, so it should
just fail unconditionally.
fanotify.c: fix typo in header inclusion

the header is included only as a guard to check that the declaration
and definition match, so the typo didn't cause any breakage aside
from omitting this check.