offload: Modify the build-machine record to accept multiple systems.

* guix/scripts/offload.scm (<build-machine>)[systems]: New field.
[system]: Accessor changed to %build-machine-system.  Default to #f.
* guix/scripts/offload.scm (build-machine-system): Wrap %build-machine-system
with a deprecation warning.
(build-machine-systems): Access the new systems field or fallback to use
build-machine-system, for backward compatibility.
(machine-matches?): Adjust.
* tests/offload.scm: Add tests...
* Makefile.am (SCM_TESTS): ...and register them.
* doc/guix.texi (Daemon Offload Setup): Update doc.