uniquefiles.mk: Let ports override directories

[0mp]

Summary

This patch makes it possible to override directory manually names when
USES=uniquefiles:dirs is used. Some ports like Ansible have to override
values of ETCDIR and DATADIR in order to preserve the defaults and POLA.
Currently, Ansible ports override ETCDIR and DATADIR after including
bsd.port.mk, which feels like a hack.

[0mp]

Idea: Alternatively, maybe we can use ?= instead of = to assign those *DIR variables so that uniquefile.mk does not assign values if the port already did that.

[0mp]

Ping.