BF: Re-write/clean up code which generates names for target masks.

The logic used to extract file basenames for each target mask specified via the --targetmasks option was flawed, and could result in a pointer underflow (i.e. tmpname[pos] = '_';, where pos == -1).

This loop (replacing all / with _ characters in the target file paths) wasn't even used anyway, as the next step in the logic would discard everything after the last _.