usbconfig: add -l option to dump_device_desc for single line output

Like pciconf -l add a -l to dump_device_desc for usbconfig which will
fold most fields into a single line. We do filter out some we think
will not be interesting in this case.

While mangling with string output in ways not always anticipated when
the program was initially written this should help to parse the
available devices for programs like fwget(8). While I had this in
mind for other bits for a while I got prompted by D48678 to finally
do it.

Sponsored by: The FreebSD Foundation
MFC after: 3 days
Reviewed by: bapt (LGTM), ziaee (man)
Differential Revision: https://reviews.freebsd.org/D48974