Fix Alt+numberpad in pterm. For a start, there was a bug whereby if

a pterm came up while Alt was down, then releasing it would cause a
^@ to be generated. Also, though, I've decided that Alt plus a
single numberpad key should not generate a low-numbered control
code, because that's too easy to do by mistake and the codes are too
powerful. Anyone who really _wants_ to create a ^C or ^D from the
numberpad can do Alt-03 or Alt-04 easily enough; two-digit codes and
more such as Alt-65 are unaffected.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2153 cda61777-01e9-0310-a592-d414129be87e