unix: SCM_CREDS: Restore passing the effective GID

cmcred_groups[0] in 'struct cmsgcred' must be the effective GID.

Note that the code in unp_addsockcred() filling up 'struct
sockcred'/'struct sockcred2' (LOCAL_CREDS/LOCAL_CREDS_PERSISTENT
options) was in fact "wrong" before 'cr_gid' was moved out of
cr_groups[], in the sense that it would transmit the effective GID
twice, both separately as 'sc_egid' and as the first element of
'sc_groups'. It is now exact, so is left unchanged, which causes
a difference in output (the effective GID is no more in 'sc_groups',
unless it is also a supplementary group) that is unlikely to affect
applications in practice.

Reviewed by: glebius
Fixes: be1f7435ef218b1d ("kern: start tracking cr_gid outside of cr_groups[]")
MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52262

(cherry picked from commit c5e920e49c0cf068da3962688cc60ab514ea1252)