dfilter: Parse character constants in lexer

Invalid character constants should be handled in the lexical scanner.

Todo: See if some code could be shared to parse double quoted strings.

It also fixes some unintuitive type coercions to string. Character
constants should be treated as characters, or maybe integers, or
maybe even throw an invalid comparison error, but coverting to a
literal string or byte array is surprising and not particularly
useful:
  '\xFF' -> "'\xFF'" (equals)
  '\xFF' -> "FF"     (contains)

Before:

    Filter: http.request.method contains "\x63"

    Constants:
    00000 PUT_FVALUE	"c" <FT_STRING> -> reg#1
    (...)

    Filter: http.request.method contains '\x63'

    Constants:
    00000 PUT_FVALUE	"63" <FT_STRING> -> reg#1
    (...)

    Filter: http.request.method == "\x63"

    Constants:
    00000 PUT_FVALUE	"c" <FT_STRING> -> reg#1
    (...)

    Filter: http.request.method == '\x63'

    Constants:
    00000 PUT_FVALUE	"'\\x63'" <FT_STRING> -> reg#1
    (...)

After:

    Filter: http.request.method contains '\x63'

    Constants:
    00000 PUT_FVALUE	"c" <FT_STRING> -> reg#1
    (...)

    Filter: http.request.method == '\x63'

    Constants:
    00000 PUT_FVALUE	"c" <FT_STRING> -> reg#1
    (...)

1 files changed, 1 insertions, 0 deletions

diff --git a/epan/ftypes/ftype-ipv4.c b/epan/ftypes/ftype-ipv4.c
index 10d893ee5e..9d51bf6c33 100644
--- a/epan/ftypes/ftype-ipv4.c
+++ b/epan/ftypes/ftype-ipv4.c
@@ -155,6 +155,7 @@ ftype_register_ipv4(void)
 		NULL,				/* free_value */
 		val_from_unparsed,		/* val_from_unparsed */
 		NULL,				/* val_from_string */
+		NULL,				/* val_from_charconst */
 		val_to_repr,			/* val_to_string_repr */
 
 		{ .set_value_uinteger = set_uinteger },	/* union set_value */