store: Support 'ssh://' URIs in 'GUIX_DAEMON_SOCKET'.

This allows 'guix' commands to talk to a remote store over SSH.

* guix/store.scm (connect-to-daemon)[connect]: Call 'resolve-interface'
for unknown URI schemes.
* guix/store/ssh.scm: New file.
* Makefile.am (MODULES): Add it.
* doc/guix.texi (The Store): Document it.  Mark remote access as
experimental.