Unify getpkt and getpkt_or_notif_sane

getpkt and getpkt_or_notif_sane are just wrappers for
getpkt_or_notif_sane_1.  This patch adds the is_notif parameter to
getpkt, with a suitable default, and removes the wrappers.

Reviewed-by: John Baldwin <jhb@FreeBSD.org>