Clean up handling of the message type.

Don't use try_val_to_str() and then never test the result to see whether
it's null.  Instead, use val_to_str_const().

Don't use proto_tree_add_uint_format_value() to emulate what
proto_tree_add_uint() does unless the string for the value isn't
computed from the value_string for the field.

Also, prepend to the Info column as soon as we have the data with which
to do so.

Change-Id: Iae9bdfc7c80cbd1ec63e1bbb8d43560adec558bd
Reviewed-on: https://code.wireshark.org/review/4113
Reviewed-by: Guy Harris <guy@alum.mit.edu>