Support new -S option to not check Subject CRCs

the Subject CRC calculation was changed in May of 2022 (trailing whitespace is
removed prior to calculation), so messages imported before this date/revision
of sbbs might be detected as having an invalid/mismatch Subject CRC. This -S
option can be used to suppress such errors (for Keyop).

Don't support /option syntax any longer in non-*nix builds (just -options).
Options are now case-sensitive (i.e. -S and -s are different options).
Bump version to 3.20.