store: 'GUIX_DAEMON_SOCKET' can now be a URI.

* guix/store.scm (%daemon-socket-file): Rename to...
(%daemon-socket-uri): ... this.
(connect-to-daemon): New procedure.
(open-connection): Rename 'file' to 'uri'.  Use 'connect-to-daemon'
instead of 'open-unix-domain-socket'.
* guix/tests.scm (open-connection-for-tests): Rename 'file' to 'uri'.
* tests/guix-build.sh: Add tests.
* tests/store.scm ("open-connection with file:// URI"): New tests.