Increase default first step threshold to 20 us.

Most PHC drivers implement stepping (ADJ_SETOFFSET) by reading the
clock, adjusting the value by the offset and writing it back. This is
not perfectly accurate and if the operation is slow (e.g. due to PCIe
latencies), the error can be in microseconds.

Increase the default first step threshold from 100 nanoseconds to 20
microseconds to step only when the initial offset is larger than
the error in the step.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Print warning message on deprecated ptp4l options.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>