rx: Hide struct rx_peer from the rest of the tree

rx: Hide struct rx_peer from the rest of the tree

The peer structure is an internal RX structure, so it shouldn't
be contained in public header files, or used by non-rx modules.

Hide the structure definition in rx_peer.h, and convert the
rx_HostOf and rx_PortOf accessors from macros into functions. This
will also help with adding support for non-IPv4 addresses to RX.

Change-Id: I790fa3193bfa629a2872fc5395d0146e76a2de9e
Reviewed-on: http://gerrit.openafs.org/7002
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>