Modify usb(4) to include usbstart and change
'usbprint' to 'usbprinter' to match commands.
Correct usbprinter mount point. Clarify and
apply conventions like smallcap acronyms
etc. Replaced 'scan' and similar with standard
'read' and 'write.' Added types for arguments.
Made references to 'hubs' and 'slots' match
naming in usb(3). Cleanup troff source.

Copied distributed usbprinter script from
/bin/usb/ to /sys/src/cmd/usb/misc/. Doc,
source, cmd now match. Changed mkfile
references to 'usbprint' to 'usbprinter'.
This patch does not remove the superseded
usbprint script.

Made usage() and doc match (they did not as
distributed) in related source, adopting 'm' and
'n' convention from usb(3). Tested (single-line)
c source changes with a clean mk in
/sys/src/cmd/usb/. Installed 386 binaries run as
expected.