It might make sense not to use pid files here but to actually test whether the
socket /var/guix/daemon-socket/socket works.

No idea how to do that with just bash.  It would be easy in Python.

Committed v4 to guix master as commits 73fbe04107d38f4561636c74d28d7a4935cbb1ef
and fe60ef998f537e0e71bbb3377d5886ad297821d0.