make DELETE-FILE respect *DEFAULT-PATHNAME-DEFAULTS*

make DELETE-FILE respect *DEFAULT-PATHNAME-DEFAULTS*

  MERGE-PATHNAMES to get an absolute pathname. (Using TRUENAME would be wrong,
  since then we would delete files pointed to by symbolic links, and not the
  symbolic links themselves -- a nasty regression that would be!)

  Also remove the "for error checking" TRUENAME call from there: unlink will
  give us an errno that tells what we need to know -- and lo! there is one
  race condition less in the system.

  Previously using relative pathnames it was possible to accidentally delete
  the wrong file.

  Fixes lp#882877.

  NB: currently DELETE-DIRECTORY and RENAME-FILE use TRUENAME with just the
      aforementioned unfortunate consequence, but I'm hesitant to change them
      during the freeze -- so dealing with this issue in them will have to
      wait a bit.