HOPKINS: Further cleanup in ComputerManager class.

Have simplified the parsing of the COMPUTAN.TXT file prior to looking at
supporting the Polish file format variant.

These change should have no functional difference, but improve the code
by removing a set-but-unused bool in the MenuItem structure, fixing a
number of repeated "magic" values to be explicit as various buffer sizes
and replacing usage of strcpy with the safer version from our Common
code etc.