Consolidate oom_check() functions

This consolidates the duplicated oom_check() functions, as well as
splitting them into OS dependant versions to avoid the #ifdef
grossness that was present in the old osdep.c version.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>