This is the one openlog() flag that toybox uses. We should probably try
to unify toybox's POSIX logger and Android-specific log at some point,
and this will help.
Also fix our behavior with an empty format string, noticed while adding
tests.
Test: treehugger
Test: adb shell logger -s foo
Change-Id: Ic027e78a460be3db83cc4c6f9946c9efa22be6e1
