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.

runtime/doc/channel.txt

@@ -633,8 +633,8 @@ ch_logfile({fname} [, {mode}])					*ch_logfile()*
 		is going on in real time.
 
 		To enable the log very early, to see what is received from a
-		terminal during startup, use |--cmd|: >
-			vim --cmd "call ch_logfile('logfile', 'w')"
+		terminal during startup, use |--log|: >
+			vim --log logfile
 <
 		This function is not available in the |sandbox|.
 		NOTE: the channel communication is stored in the file, be

runtime/doc/starting.txt

@@ -344,6 +344,12 @@ a slash.  Thus "-R" means recovery and "-/R" readonly.
 		Example: >
 			vim -V20vimlog foobar
 <
+--log {filename}					*--log*
+		Start logging and write entries to {filename}.
+		This works like calling `ch_logfile({filename}, 'a')` very
+		early during startup.
+		{only available with the +channel feature}
+
 							*-D*
 -D		Debugging.  Go to debugging mode when executing the first
 		command from a script. |debug-mode|
@@ -564,6 +570,7 @@ a slash.  Thus "-R" means recovery and "-/R" readonly.
 		{scriptout} cannot start with a digit.
 		If you want to record what is typed in a human readable for
 		you can use |ch_logfile()|, It adds "raw key input" lines.
+		Also see |--log|.
 
 							*-W*
 -W {scriptout}	Like -w, but do not append, overwrite an existing file.