dirmngr: Add doc for faked-system-time
* dirmngr/dirmngr.c (gpgrt_opt_t): Use string for oFakedSystemTime. (oFakedSystemTime): Use similar conversion as gpgsm has. * dirmngr/dirmngr.texi (faked-system-time): Document it.
For testing X509 certificates this is usually required and
then confusing that the example from the gpgsm man page
does not work for dirmngr.