sd-event: add support for CLOCK_BOOTTIME

This requires a very recent kernel (3.15), so care should be taken
when using this functionality.
time-util: add clock_boottime_or_monotonic

CLOCK_BOOTTIME is not supported by timerfd on older kernels, so for the time beeing,
use this helper instead which will fallback to CLOCK_MONOTONIC if CLOCK_BOOTTIME is
not supported.
libsystemd-network: use CLOCK_BOOTTIME instead of CLOCK_MONOTONIC where possible

The timeouts in the networking library (DHCP lease timeouts and similar) should not be affected
by suspend. In the cases where CLOCK_BOOTTIME is not implemented, it is still safe to fallback to
CLOCK_MONOTONIC, as the consumers of the library (i.e., networkd) _should_ renew the leases when
coming out of suspend.