Add some attribute/TLV length checks.

Add some attribute/TLV length checks.

Make sure we don't run past the end of a BGP attribute or LDP TLV when
dissecting the attribute/TLV.

Make some of the code do a bit more of a "step the pointer through the
data"-style dissection; that was done while debugging the changes in
question.  It also fixes up some code to not check for more data than
should actually be there.

Update references to RFC 4906 from the draft, and note that RFC 4447
replaces it.