remove the DOS_FILE and MAC_FILE flags, as they're only used in files.c,

remove the DOS_FILE and MAC_FILE flags, as they're only used in files.c,
and replace them with a static file_format enum; change the
openfilestruct structure accordingly in order to handle this; also add a
few miscellaneous cleanups

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2054 35c25a1d-7b9e-4130-9fde-d3aeb78583b8