Add new-style compression that skips matching data.

Adding new-style compression that only compresses the literal data that
is sent over the wire and not also matching file data that was not sent.
This new-style compression is compatible with external zlib instances,
and will eventually become the default (once enough time has passed that
all servers support the --new-compress and --old-compress options).

NOTE: if you build rsync with an external zlib (i.e. if you specified
configure --with-included-zlib=no) you will ONLY get support for the
--new-compress option!  A client will treat -z as uncompressed (with a
warning) and a server will exit with an error (unless -zz was used).
Change args to file_checksum() to prepare for future changes.