dhcpclient: Load dictionary.dhcp from DICTDIR.

Load dictionary.dhcp from DICTDIR instead of RADDBDIR in dhcpclient.c,
as it is found only in the former.

This fixes the following error printed when invoking dhcpclient:

    Failed reading dictionary.dhcp: dict_init: Couldn't open dictionary
    "/etc/raddb/dictionary.dhcp": No such file or directory