Unicode cleanup phase 2: we now reintroduce the ability to enter a

Unicode cleanup phase 2: we now reintroduce the ability to enter a
numeric code page, and also reinstate the direct-to-font zero
translation mode (but now under an actual _name_ rather than blank).
Also add CP437 to the list since at least one expatriate DOS user
wanted it; also select a sensible ISO or KOI codepage based on the
system locale.

git-svn-id: svn://svn.tartarus.org/sgt/putty@1230 cda61777-01e9-0310-a592-d414129be87e