Modify slapd set user/group support to use -u/-g instead -U/-G.

Moved -u (udp) to -c (connectionless).  Connectionless is depreciated
and undocumented.
Added tests for set{,e}{u,g}id to configure.  Use existance of
setuid/setgid to enable feature.  Use seteuid/setegid only if
they exist.
Fixed minor minor leak of username/groupname if specificied
more than once.  (This should actually be an error)