In support of message bases with vote-messages and poll-closures:

smb_msg_type() returns an enum smb_msg_type inferred from the combination
of attribute flags specified (should match msghdr_t.type).

smb_msg_count() calculates the number of message index records of the
specified type(s) (a bit-field) by reading the message base index file. This
value should only used for info/display purposes, so no locking is performed
to return the result as fast as possible.

This allows an accurate number of "posts" (posted *messages*) to be queried
and displayed to a user when appropriate (e.g. instead of
smb.status.total_msgs or just the size of the index file divided by the size
of an index record). In that case, since vote messages and poll closures
aren't displayed as messages to users, exclude those from the count to reduce
confusion. For message bases that don't support voting, this scheme is not
necessary.