patch 8.2.5056: the channel log only contains some of the raw terminal output

Problem:    The channel log only contains some of the raw terminal output.
Solution:   Add the "o" flag to log all terminal output.  Use it for "--log".

runtime/doc/starting.txt

@@ -346,7 +346,7 @@ a slash.  Thus "-R" means recovery and "-/R" readonly.
 <
 --log {filename}					*--log*
 		Start logging and write entries to {filename}.
-		This works like calling `ch_logfile({filename}, 'a')` very
+		This works like calling `ch_logfile({filename}, 'ao')` very
 		early during startup.
 		{only available with the +channel feature}