So util.c has a few functions to pretty print interface information
like media and link state. These functions are highly system specific and
only used by kroute.c and in bgpctl to show kroute internals.
This diffs creates a ctl_show_interface struct used in the imsgs between
bgpd and bgpctl. The media and link state are passed as strings and so
bgpd/kroute.c can create these. This way the code can be moved out of util.c
and moved in the more OS specific code.