save without crash-corrupting log file

4.21: adde/journaling/save without crash-corrupting log file:
. when saving the user's commands log,
each save needs to re-open and re-close the file;
because, a crash when a file is open might corrupt it .
. this could be done using a pair of files;
or we could use the platform's way of writing to a file buffer;
and then atomically set the save-file's pointer to point at buffer;
or (copy saved; append the command; close the copy;
    rename copy as saved).