Add support for selectively customized UNITDATA files

Since it is desirable for modders to uniquely name their customized
UNITDATA files to avoid clashes, Raw2Txt and Txt2Raw have been updated
to allow converting files where the filename only has to start with the
word UNITDATA but can then contain additional words.

See also: http://forums.runicgames.com/viewtopic.php?f=47&t=48200