Add --forground option

Which improves interaction with init systems which require
programs not daemonizing.  E. g. systemd.

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
irq_update_stats: 'queue' was sometimes left uninitialized

Seen on some laptop:

 Mar  9 12:11:36 knotty irqd: interface.c:137: BUG

Which was due to the wireless NIC (named 'wlp2s0') used:

 17: 1591172  538   IO-APIC-fasteoi   ehci_hcd:usb1, wlp2s0

if_queue_by_name() was called with an uninitialized 'queue'
value in this case, letting it go BUG() in if_qeue().

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>