Change the semantics of 'FontSpec' so that it's a dynamically

Change the semantics of 'FontSpec' so that it's a dynamically
allocated type.

The main reason for this is to stop it from taking up a fixed large
amount of space in every 'struct value' subunion in conf.c, although
that makes little difference so far because Filename is still doing
the same thing (and is therefore next on my list). However, the
removal of its arbitrary length limit is not to be sneezed at.

git-svn-id: http://svn.tartarus.org/sgt/putty@9314 cda61777-01e9-0310-a592-d414129be87e