journald: allow additional payload in server_driver_message

journald: allow additional payload in server_driver_message

The code to format the iovec is shared with log.c. All call sites to
server_driver_message are changed to include the additional "MESSAGE="
part, but the new functionality is not used and change in functionality
is not expected.

iovec is preallocated, so the maximum number of messages is limited.
In server_driver_message N_IOVEC_PAYLOAD_FIELDS is currently set to 1.

New code is not oom safe, it will fail if memory cannot be allocated.
This will be fixed in subsequent commit.