config/sys.config: group pfcp_peer params into a map
Following the same pattern as sctp_{client,server}, group the flat pfcp_loc_addr/pfcp_rem_addr environment variables into a pfcp_peer map. The old flat keys are still supported for backwards compat.
Changes: * osmo_s1gw_sup: add pfcp_cfg(), merging legacy flat keys with the new pfcp_peer map (new takes priority); store the resolved config back via set_env(pfcp_peer, ...) so all consumers see a single canonical map * pfcp_peer: change start_link/2 to start_link/1 taking a cfg() map; simplify init() using sctp_common:parse_addr/1; add cfg() type * rest_server: read pfcp laddr/raddr from the pfcp_peer map
