instead of starving the audio driver, play 'silence'

according to libao developers, starving the driver can lead to
unwanted audio artifacts or no audio at all, depending on the specific
driver in use. so we have to generate and play silence whenever no beep
is wanted.
implement 'filebased' replay mode

instead of capturing on a monitor device, an existing pcap dump file
can be passed as argv[1], which is then used to simulate.

this makes it possible to debug/develop the program without active
monitor device or root perms.