Clean up protocol ID printing.

Clean up protocol ID printing.

If getprotobynumber() doesn't return a valid protocol name, just print
the protocol number.

Don't bother with setprotoent() and endprotoent() - none of the other
dissectors using getprotobynumber() use them and, if it's a useful
optimization, we should do the optimization in a common routine for all
callers (or do some other optimization, e.g. reading the entire protocol
list and building a table in which we can do a quick lookup).