Define `AF_UNIX' only when Unix-domain sockets are supported.

* libguile/socket.c (scm_init_socket): Defined `AF_UNIX' only when
  `HAVE_UNIX_DOMAIN_SOCKETS' is defined.
  Reported by Eli Zaretskii <eliz@gnu.org>.
tests: Use double quotes around shell arguments, for Windows.

* test-suite/standalone/test-system-cmds (test-system-cmd): Use double
  quotes around shell arguments.
  Reported by Eli Zaretskii <eliz@gnu.org>.
tests: Don't rely on $TMPDIR and /tmp on Windows.

* test-suite/standalone/test-unwind.c (check_ports)[__MINGW32__]: Use
  $TEMP, $TMP, or / as the value for TMPDIR.
  Patch by Eli Zaretskii <eliz@gnu.org>.
tests: Skip FFI tests that use `qsort' when it's not accessible.

* test-suite/tests/foreign.test ("procedure->pointer")[qsort]: Wrap in
  `false-if-exception'.
  ["qsort", "qsort, wrong return type", "qsort, wrong arity"]: Throw
  'unresolved when QSORT if #f.
  Reported by Eli Zaretskii <eliz@gnu.org>.
tests: Remove symlink only when it exists.

* test-suite/tests/filesys.test: Delete (test-symlink) only if it
  exists---i.e., not on Windows.
  Reported by Eli Zaretskii <eliz@gnu.org>.
Augment `.dir-locals.el'.
tests: Don't rely on `scm_call_2' being visible.

* test-suite/tests/coverage.test ("procedure-execution-count")["called
  from C"]: Throw 'unresolved when `scm_call_2' cannot be resolved.
  Reported by Eli Zaretskii <eliz@gnu.org>.
Fix #ifdefery for `setegid'.

* libguile/posix.c (scm_setegid): Change to #ifdef HAVE_SETEGID.
Fix bug in remqueue in threads.c when removing last element.

Reported and debugged by Andrew Gaylard <ag@computer.org>.

* libguile/threads.c (remqueue): When removing the last element from a
  queue with more than one element, set (car q) to the previous element.

* THANKS: Thank Andrew Gaylard.