Treat SI:localuser, SI:localgroup as secure

Since at least X11R6.8.0 there is a ServerInterpreted family of X host access. On Ubuntu 9.10, [send] does not work because by default there are SI:localuser authorizations in the authorized hosts list. The same is apparently true of Fedora 8 and newer.

SI:localuser and SI:localgroup authorizations can be as secure as Xauth-only authorizations.

This patch makes [send] continue to work even if SI:localuser and SI:localgroup authorizations appear in the authorization list.

'xhost +' and 'xhost + localhost' still make [send] stop with an error about the X server being insecure.

Before applying this patch, revert tkUnixSend.c r1.25 on TRUNK and r1.20.2.1 on the tk8.5 branch; that change was wrong. It only allows a single SI: authorization. Ubuntu 9.10 has 3 by default (this user, gdm, root).