Add JS methods for vetting filenames (e.g. for upload by users)

system.illegal_filename() - check if contains illegal chars/sequences
system.safest_filename() - check if contains only safest chars
system.allowed_filename() - check if meets criteria from SCFG->File Options
system.check_filename() - check if legal and meets configured criteria and is not in file.can
bbs.check_filename() - ditto, except will display badfile.msg as appropriate

Now scripts can utilize the sysop-controlled filename criteria and security of the BBS.