Use "-H", not "-h", for the 802.11s option, and improve error reporting.

Use "-H", not "-h", for the 802.11s option, and improve error reporting.

I was rudely surprised to find that "tcpdump -h" wasn't printing a usage
message, and I'm the person who *added* the "-h" option.  Make it "-H",
and add an explicit "-h" option to print a usage message, so nobody else
makes the same mistake.  Also, don't clear opterr, so that if you give
an illegal command-line option, you get an explanatory error message.