Get rid of some other uses of ethertype() for protocols that don't have

Get rid of some other uses of ethertype() for protocols that don't have
a trailer.

Clean up the NHRP dissector's handling of the protocol ID to fully
implement RFC 2332 (and mention that 2332 is the RFC for NHRP).  Don't
compute the checksum unless we have all the packet data, and don't use
the captured length as the actual packet length.  Check for an invalid
extension offset value.  Get rid of some unneeded
tvb_ensure_bytes_exist() calls.  Dissect - and set the columns -
regardless of whether we're building the protocol tree.  Mark the packet
in error reports as an error packet.

svn path=/trunk/; revision=28273