flock: improve timeout handling

flock: improve timeout handling

Signal ALRM raised by the timer, and the timer only, will be considered
as a timeout criteria.

Secondly time interval is made to use monotonic clock.  Documentation of
ITIMER_REAL is unclear whether that time is affected various sources of
clock skew, or does it even tick when system is suspended.

Reviewed-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>