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.

src/basic/log.h

@@ -26,6 +26,7 @@
 #include <stdbool.h>
 #include <stdlib.h>
 #include <sys/signalfd.h>
+#include <sys/socket.h>
 #include <syslog.h>
 
 #include "sd-id128.h"
@@ -127,6 +128,15 @@ int log_oom_internal(
                 int line,
                 const char *func);
 
+int log_format_iovec(
+                struct iovec *iovec,
+                unsigned iovec_len,
+                unsigned *n,
+                bool newline_separator,
+                int error,
+                const char *format,
+                va_list ap);
+
 /* This modifies the buffer passed! */
 int log_dump_internal(
                 int level,