Hush a compile warning

Function only used by libuninameslist to convert tags, exclude if unused.
Allow user to use - or -- for options but only - for stdin or single chars

Problem was found while trying "fontforge --lang=ff --script -"
If using an option like -c, linux expects only a single - and not --c.
Meanwhile earlier code wasn't able to accept --script due to CheckIsScript.
These changes allow backwards compatibility with single or double - on
words since OSes like unix standardize on 2, while windows normally as 1.

CheckIsScript changed arg to pt to make it more clear since several vars
are labeled closely as arcc, argv. Also ensure -- not changed to -