Made handling of byte strings in scanner and parser much simpler,

Made handling of byte strings in scanner and parser much simpler,
improving size of grammar and creating the possibility of dfilter_compile
reporting errors back to user. In this case, if an ETHER variable is
compared against a byte string that is not 6 bytes, an error condition is
flagged appropriately. I have not put in the code to conver that error flag
to a message to the user, but that's what I'm working on next.

Also, fixed sample debug session in README to show correct gdb prompt.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@522 f5534014-38df-0310-8fa8-9805f1628bb7