Proper support for using the font's own character encoding. If we

know what that encoding actually is, we can do our best to support
additional charsets (VT100 linedrawing, SCO ACS, UTF-8 mode) using
the available characters; if we don't, we fall back to a mode where
we disable all Unicode cut-and-paste and assume any Unicode
character is undisplayable.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2413 cda61777-01e9-0310-a592-d414129be87e