Log Message:
roundcube: use PKG_SYSCONFDIR for plug-in configuration files

This moves the configuration files for Roundcube plug-ins to $PKG_SYSCONFDIR,
where they should belong instead of $RC_DIR/$PLUGIN_DIR.

This works without any further patches, because Roundcube falls back to
RCUBE_CONFIG_DIR.'/'.$this->ID . '.inc.php' for plug-ins basically.

Bumps PKGREVISION for the plug-ins using ../../mail/roundcube/plugins.mk
(enigma, password, and zipdownload).

Tested on NetBSD/amd64.

OK taca@