get rid of doschar_table[]

get rid of doschar_table[]

If I'm not completely blind, then check_dos_char is *only* used in the case
when we can't mmap() valid.dat. To me this looks as if we initialize the 65536
bits in doschar_table[] with check_dos_char_slowly, use it once to initialize
valid_table[] and *never* use them again. I think there's no point in keeping
these 8k of modified memory around for an unlikely case (no "valid.dat") and
even that only to use it exactly once.