Improve RAID level parsing

Improve RAID level parsing

Check for numeric levels before checking for single-character ones
("c" and "C") such that a) invalid synopsis (no "-l") is detected as such
	# bioctl -c 10 vnd0 softraid0
	bioctl: Invalid RAID level
	# ./obj/bioctl -c 10 vnd0 softraid0
	usage: bioctl ...

and b) ten is correctly treated as valid but unsupported level:
	# bioctl -c 10 -l vnd0a softraid0
	bioctl: Invalid RAID level
	# ./obj/bioctl -c 10 -l vnd0a softraid0
	bioctl: unsupported RAID level

Uppercase the abbreviation while here.

Feedback OK tb