More uniform safe/illegal/allowed filename (for upload) determination

sbbs_t::checkfname() now checks the file.can too.
new filedat.c functions:
- safest_filename() - not currently used
- illegal_filename() - returns true for a highly-suspicious (e.g. hack attempt) filename
- allowed_filename() - returns true if the filename is good for upload (assumed to be already checked to be legal as well).

Importantly, filenames beginning or ending in a '.' are now unallowed:
- 'dot files' are hidden (by default) on *nix
- files ending in a '.' are problematic on Windows