Conditionally install (if_)?(otus|rsu).4, otusfw.4, rsufw.4, and urtwn.4 if
MK_USB != no

Add the manpages to OptionalObsoleteFiles.inc

As a side-effect, this also fixes installworld with MK_USB == no

X-MFC with: r290128
Sponsored by: EMC / Isilon Storage Division