log: add a mode where we open the log fds for every single log message

This we can then make use in execute.c to make error logging a bit less
special when preparing for process execution, as we can still log but
don't have any fds open continously.

src/basic/log.h

@@ -303,6 +303,7 @@ void log_received_signal(int level, const struct signalfd_siginfo *si);
 
 void log_set_upgrade_syslog_to_journal(bool b);
 void log_set_always_reopen_console(bool b);
+void log_set_open_when_needed(bool b);
 
 int log_syntax_internal(
                 const char *unit,