Rework default pathname defaults and working directory pathname.

Rework default pathname defaults and working directory pathname.

New logic:

- *default-pathname-defaults* is just #!default, which as an argument
  to pathname operations has the same effect as not passing it and
  implying the default pathname defaults parameter instead.

- (param:default-pathname-defaults) can be set to #f (default),
  meaning use whatever (working-directory-pathname) returns

- wrkdir.scm no longer depends on the default pathname defaults
  mechanism at all (although it is still kind of broken: it should be
  a thread-local file descriptor fit for use with openat &c.)

This way nothing gets stored in a band.

If the extra parsing turns out to be slow, we can the working
directory pathname in a string that we clear with a secondary GC
daemon, but let's wait for a measurement of slowness before
bothering.