camcontrol fwdownload minor improvements

Minor improvements to the fwdownload code suggested by chs@:
o Print the path_id/target we're rescanning so it's not invisible
o No need for XPT_GDEVLIST, all the info is filled in. Remove sending it

as well as a comment related to it from a mistaken observation. libcam
always fills these in properly, so use those for the ccb path/target.

o Don't leak /dev/xpt fd in success cases.
o Rename fw_rescan_lun to fw_rescan_target and pass sim_mode to

only print path_id and target_id info.

Reviewed by: chs@
Fixes: 9835900cb95bcd068774934961fb1419719d595b
Sponsored by: Netflix
MFC After: 1 week
Differential Revision: https://reviews.freebsd.org/D34348