Add options to LogAction() so it can append (instead of truncate) and write without...

Add options to LogAction() so it can append (instead of truncate) and write without buffering.

Was so happy to discover LogAction() but disappointed that it truncates
the file each time the process starts, and apparently doesn't flush the
file upon exiting.  With two new optional args, the file can be appended
rather than truncated and buffering disabled.  The defaults remain as
before, which is to say to truncate and to buffer.