Changes/additions to input character set handling:

Changes/additions to input character set handling:

 - After discussion with Simon, change the default input charset back to ASCII,
   rather than trying to work it out from the locale, for the sake of promoting
   .but file portability.

 - Add a new command-line option "--input-charset=csname", which overrides the
   ASCII default for all input files (since there's no other way to use a
   non-ASCII-compatible input file).

 - Output a warning if -Cinput-charset:foo is specified that it has no effect.

 - Update the docs to match all this. Also try to clarify some other things in
   this area that caught me out.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@5332 cda61777-01e9-0310-a592-d414129be87e