For convenience of debugging, and perhaps some real convenience at

For convenience of debugging, and perhaps some real convenience at
some point too: introduce a bunch of environment variables which can
override Unix PuTTY's usual idea of where to find its dotfiles.
Setting PUTTYDIR moves the entire ~/.putty directory; setting
PUTTYSESSIONS, PUTTYSSHHOSTKEYS or PUTTYRANDOMSEED move specific
things within that directory.

While I'm here, also be prepared to fall back to password file
lookups if $HOME is undefined (though we still use $HOME in
preference when it is defined, because that's polite and useful).
Also, on general principles, tweak the make_filename() function
prototype so it doesn't rely on fixed-size buffers.

git-svn-id: svn://svn.tartarus.org/sgt/putty@7934 cda61777-01e9-0310-a592-d414129be87e