zone.lisp: Fix default output directory.

The code used to capture `*default-pathname-defaults*' in
`*zone-output-path*' at load time, which (varying by implementation)
might indicate the calling process's working directory, or might be the
directory from which the file was loaded.  The latter is obviously
wrong, so we adopt the convention in `zone-file-name' that if
`*zone-output-path*' is nil then we use `*default-pathname-defaults'
from the time of the call.