Rather more natural (if much more complex) Mac Filename implementation.

Rather more natural (if much more complex) Mac Filename implementation.
Filenames are represented as a FSSpec, which is converted to and from an
alias record ('alis' resource) when saving and loading sessions.
.
It might be an idea to allow in-core Filenames to contain alias records too,
so that they can refer to directories that don't exist on the current system,
but that requires Filenames to be dynamically allocated, which is likely to be
a pain.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2771 cda61777-01e9-0310-a592-d414129be87e