Arguments to ctype functions are required to be either EOF or representable

Arguments to ctype functions are required to be either EOF or representable
as unsigned char.  This means that passing in a bare char is incorrect on
systems where char is signed.  Sprinkle some appropriate casts to prevent
this.

git-svn-id: http://svn.tartarus.org/sgt/putty@8406 cda61777-01e9-0310-a592-d414129be87e