patch 8.2.4742: there is no way to start logging very early in startup

Problem:    There is no way to start logging very early in startup.
Solution:   Add the --log argument.  Include the date in the start message in
            the log file.  Avoid a duplicate message when forking.  Log an
            executed shell command.
diff --git a/src/channel.c b/src/channel.c
index f8a8194..e971e17 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -178,7 +178,10 @@
 
     if (log_fd != NULL)
     {
-	fprintf(log_fd, "==== start log session ====\n");
+	fprintf(log_fd, "==== start log session %s ====\n",
+						 get_ctime(time(NULL), FALSE));
+	// flush now, if fork/exec follows it could be written twice
+	fflush(log_fd);
 #ifdef FEAT_RELTIME
 	profile_start(&log_start);
 #endif