services: ntpd: Add 'allow-large-adjustment?' knob.

Suggested by Leo Famulari.

* gnu/services/networking.scm (<ntp-configuration>)[allow-large-adjustment?]:
New field.
(ntp-shepherd-service): Honor it.
(ntp-service): Add #:allow-large-adjustment? and honor it.
* doc/guix.texi (Networking Services): Document it.