Update to the 'gentle SIGHUP' patch. (ITS#1679)

- Let write operations return unwilling-to-perform after
  'gentle shutdown' has been initiated.
- Change -1 to 2 in slapd_gentle_shutdown and slapd_shutdown, since
  sig_atomic_t can be unsigned (ITS#1736).  The 'gentle SIGHUP' patch
  is older than ITS#1736 but was applied later, so it reintroduced
  the problem.

Hallvard B. Furuseth <h.b.furuseth@usit.uio.no>, June 2002.