container: _nmsg_container_serialize(): fix warning on old gcc's

nmsg/container.c: In function '_nmsg_container_serialize':
nmsg/container.c:122: warning: 'len_wire' may be used uninitialized in this function

on "gcc (GCC) 4.2.1 20070831 patched [FreeBSD]"
xsio: fix warnings on old gcc's

nmsg/xsio.c: In function 'nmsg_output_open_xs_endpoint':
nmsg/xsio.c:151: warning: 'socket_type' may be used uninitialized in this function
nmsg/xsio.c: In function 'nmsg_input_open_xs_endpoint':
nmsg/xsio.c:106: warning: 'socket_type' may be used uninitialized in this function

on "gcc (GCC) 4.2.1 20070831 patched [FreeBSD]"
nmsg/base/pkt: ensure that the "pkt" type cannot be used with pcap sources that are not DLT_EN10MB