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.
(This used to be commit 0bfea7259e4091c6d914528a0e65fc10d8a56fb8)