Fix games/golddig in the face of picky compilers

C (and linkers) has changed a little since 2001, so patch up the
places where "extern" is needed, add <string.h> and missing
cases in switch() to silence warnings. golddig runs nearly as
well as on my Apple //c (see games/kgoldrunner for a newer take
on the same concept, though)

Not taking maintainership, I just spotted this among the recent
fallout-cleanup and thought "I can do that while I wait for the
potatoes to boil."