New function: smb_msghdr_str_list() which returns a message header as a string

list (str_list_t) where each string is a header field (variable or fixed) in
"<name> <value>" format. This format more closely resembles the output
of the sbbs_t::msghdr() function which is used to dump message headers
(e.g. for sysops/sub-ops).

I intend to replace the duplicate logic in sbbs_t::msghdr() with this new
function.

I would have liked to have used a list of named strings (named_string_t), but
I don't yet have a library to alloc/add items to a named string list.