Move processing of startup-packet switches and GUC settings into InitPostgres,

Move processing of startup-packet switches and GUC settings into InitPostgres,
to fix the problem that SetClientEncoding needs to be done before
InitializeClientEncoding, as reported by Zdenek Kotala.  We get at least
the small consolation of being able to remove the bizarre API detail that
had InitPostgres returning whether user is a superuser.