gnu: lsh: Move to (gnu packages ssh)

* gnu/packages/lsh.scm: Remove.  Move 'lsh and liboop' to...
* gnu/packages/ssh.scm (liboop, lsh): ... here.  New variables.
* gnu/services/ssh.scm, gnu/services/base.scm: Adjust accordingly.
* gnu/local.mk (GNU_SYSTEM_MODULES): Adjust accordingly.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>