dirmngr: Initialize cache from sysconfig dir

* dirmngr/certcache.c (cert_cache_init): Load certificates
from sysconfig dir instead of the homeidr.
* dirmngr/dirmngr.c (main): Removed parsing of obsolete
homedir_data option.
* dirmngr/dirmngr.h (opt): Removed homedir_data.
* doc/dirmngr.texi: Update and clarify certs directory doc.

Using the homedir for extra-certs and trusted-certs makes
little sense when dirmngr is used with a caller that
manages it's own store of certificates and can
provide those through the SENDCERT command.
You can use trusted-certs and extra-certs to provide
users with a base of locally available certificates that are
not already in store of the applications.