Fix EXEC BACKEND + Windows builds for group privs

Fix EXEC BACKEND + Windows builds for group privs

Under EXEC BACKEND we also need to be going through the group privileges
setup since we do support that on Unixy systems, so add that to
SubPostmasterMain().

Under Windows, we need to simply return true from
GetDataDirectoryCreatePerm(), but that wasn't happening due to a missing
 #else clause.

Per buildfarm.