FIX: accept arbitrary delimiter while message type is being parsed

Make fix_msg_type_parse() usable for parsing FIX/FAST streams, for
example, by adding a "delim" parameter that specifies the message field
delimiter.

Signed-off-by: Marat Stanichenko <mstanichenko@gmail.com>
[ penberg@kernel.org: changelog ]
Signed-off-by: Pekka Enberg <penberg@kernel.org>