Find the line ending using tvb_find_line_end().

Find the line ending using tvb_find_line_end().

tvb_find_line_end(), unlike a tvb_find_guint8() looking for an LF,
returns a length that *doesn't* include the line ending, *regardless* of
whether the line ends with CR-LF or just LF, so the query string we
extract is just the query, without any of the line ending.

Update some comments while we're at it to note that the "next_offset"
pointer argument to tvb_find_line_end() and tvb_find_line_end_unquoted()
can be NULL, in which case the offset *past* the line ending isn't
returned.  (We pass tvb_find_line_end() NULL in the aforementioned call,
because, in that particular case, we don't care about the next line.)

Change-Id: I1c9746e32c61a79f8cb636d577a2e14a07ecab17
Reviewed-on: https://code.wireshark.org/review/35566
Petri-Dish: Guy Harris <guy@alum.mit.edu>
Tested-by: Petri Dish Buildbot
Reviewed-by: Guy Harris <guy@alum.mit.edu>