Clean up printing of LLC packets.

Clean up printing of LLC packets.

Don't print LLC header information for SNAP packets; if we have a SNAP
header, just call snap_print() and return its return value, regardless
of whether it's 1 or 0, don't fall into the code to print raw LLC header
information - and don't print it with -e, either.

If llc_print() returns 0, just call the default packet printer, don't
print the MAC-layer header or the extracted ethertype - llc_print() will
print the source and destination MAC addresses and whatever type
information is in the LLC or SNAP headers.

If we don't know the DSAP/LSAP, and it's an information frame (numbered
or not) and not an XID frame, return 0, so that we give a hex dump of
the raw payload.

In addition, print the length when printing SNAP header information with
-e.