powerd: Linger a little bit if battery life is low

powerd: Linger a little bit if battery life is low

So that we could have time to plugin AC line.

And in addition to logging messages on console, alarming audio
alert, since most laptop users will use X; they will not notice
the console messages.  Audio alert is enabled by default and
could be disabled by -Q.

While I'm here, fix powerd.8 and allow disabling battery life
monitoring if 0 is passed to -P.

Suggested-by: seallyhs