guix system: 'reconfigure' disallows downgrades by default.

guix system: 'reconfigure' disallows downgrades by default.

This is similar to what 9744cc7b4636fafb772c94adb8f05961b5b39f16 did for
'guix pull'.

* guix/scripts/system/reconfigure.scm (ensure-forward-reconfigure)
(warn-about-backward-reconfigure, channel-relations)
(check-forward-update): New procedures.
* guix/scripts/system.scm (perform-action): Add #:validate-reconfigure.
Call 'check-forward-update' when ACTION is 'reconfigure.
(%options, show-help): Add "--allow-downgrades".
(%default-options): Add 'validate-reconfigure' key.
(process-action): Pass #:validate-reconfigure to 'perform-action'.
* doc/guix.texi (Invoking guix system): Document 'guix system describe'
more prominently, and document '--allow-downgrades'.