Fix auto-detection of dictionaries for kde4-l10n ports.

With the removal of '.include <bsd.port.options.mk>' and
'.include <bsd.port.pre.mk>' PORTSDIR is no longer defined. Since Ports is
(almost) never based at the root system the checks will always fail to find a
port.

Fix this by using relative searches for the dictionary ports. Giving PORTSDIR
a conditional assignment of /usr/ports was rejected due to lack of precedent.

While here teach kde4-l10n about en_GB.