maint.mk: provide "make upload" to ease uploading

maint.mk: provide "make upload" to ease uploading

See <http://lists.gnu.org/archive/html/bug-gnulib/2012-08/msg00028.html>.
Do not depend simply on the current $(VERSION), as there may have been
new commits since the tarball generation.  Rather, rely on $(RELEASE),
as "make release-commit" already does.

For consistency, add "make release RELEASE='X.Y TYPE'" as an alias for
"make TYPE".

* top/maint.mk (upload_command, upload, release): New.
(RELEASE_TYPE): If undefined, default to the second word of $(RELEASE).
(VERSION): first word of $(RELEASE) is always right.
(emit_upload_commands): Adjust.
* top/README-release: Update.