libsystemd-network: use CLOCK_BOOTTIME instead of CLOCK_MONOTONIC where possible

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.