agent,dirmngr: Re-read the user specified config file.

* agent/gpg-agent.c (reread_configuration): Use a two-part config
file.
* dirmngr/dirmngr.c (reread_configuration): Ditto.

If --options is used to to set a specific options file, this file and
not the default file needs to be re-read on SIGHUP.

• GnuPG-bug-id: T4788