main: introduce -D cmdline option, don't auto-scan for devices

Add a -D command line option which skips auto-detection of devices upon
startup. This can speedup program startup for setups with known devices,
and allows to skip the scan for troubled drivers which might break the
startup phase.

This -D option can be combined with -d specs, either presence or absence
of -d is acceptable when -D is specified. Users still can interactively
scan for and use devices after program startup.

This resolves bug #1116.