Merge branch 'dm/compat-s-ifmt-for-zos'

Long overdue departure from the assumption that S_IFMT is shared by
everybody made in 2005.

* dm/compat-s-ifmt-for-zos:
compat: convert modes to use portable file type values

diff --cc Makefile
Simple merge

diff --cc cache.h
Simple merge

diff --cc configure.ac
Simple merge

diff --cc git-compat-util.h
Simple merge