publish: Add support for lzip.

publish: Add support for lzip.

* guix/scripts/publish.scm (show-help, %options): Support '-C METHOD'
and '-C METHOD:LEVEL'.
(default-compression): New procedure.
(bake-narinfo+nar): Add lzip.
(nar-response-port): Likewise.
(string->compression-type): New procedure.
(make-request-handler): Generalize /nar/gzip handler to handle /nar/lzip
as well.
* tests/publish.scm ("/nar/lzip/*"): New test.
("/*.narinfo with lzip compression"): New test.
* doc/guix.texi (Invoking guix publish): Document it.
(Requirements): Mention lzlib.