usbconfig: documentation fixes, mainly for -i option

usbconfig: documentation fixes, mainly for -i option

* in usage(), clearly mark -i interface as optional
* both, -u busnum and -a devaddr are optional as well
* various minor man page fixes
* clearly mark those two commands that actually use -i ifaceidx
* remove unused bitfield tag got_iface
* fix indentation level according to review comment

Differential Revision:	https://reviews.freebsd.org/D33579/
Reviewed by:	hselasky
MFC after:	2 weeks

(cherry picked from commit cae1884d4791726f5acf5d64bba9a3583b63e38b)