compat: convert modes to use portable file type values

compat: convert modes to use portable file type values

This adds simple wrapper functions around calls to stat(), fstat(),
and lstat() that translate the operating system's native file type
bits to those used by most operating systems.  It also rewrites the
S_IF* macros to the common values, so all file type processing is
performed using the translated modes.  This makes projects portable
across operating systems that use different file type definitions.

Only the file type bits may be affected by these compatibility
functions; the file permission bits are assumed to be 07777 and are
passed through unchanged.

Signed-off-by: David Michael <fedora.dm0@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>