Makefile: update defaults for modern Cygwin

Makefile: update defaults for modern Cygwin

Now that Cygwin 1.7.x has enabled lots of new features, and Cygwin 1.5
is no longer actively supported by the Cygwin mailing lists, we might
as well update the defaults to cater to those new features.

NO_TRUSTABLE_FILEMODE is only necessary on FAT drives; the Cygwin
community recommends NTFS drives, but there is still too much use
for FAT to switch the default.  Likewise, UNRELIABLE_FSTAT is probably
file-system specific, but worth keeping unchanged.

This commit does not change the default for NO_MMAP, although definitive
proof of whether this option is necessary is lacking.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>