non-zero-length string, will touch/create that file if outgoing data has
been processed.

Intended to allow the mailer to scan as soon as outgoing data is available
rather than having it wait for the next normally scheduled scan.

Note, if using this feature as the *only* way to trigger a scan (not
reccomended), any other sources of outgoing data (such as hatching files,
interBBS doors, etc) will also need to touch this semaphore or they will
not be sent until sbbsecho processes outgoing data.