missing: add onestep sync to missing.h

this patch uses grep to test whether the net_tstamp.h header has
HWTSTAMP_TX_ONESTEP_SYNC flag defined. If it doesn't then we can simply define
it with the correct value. This works because proper drivers should just report
that the value is not allowed if they don't support onestep mode. This is the
cleanest way to ensure that linuxptp will still work on kernels which have not
defined the one step flag, and also works for any distributions which backport
the feature.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
phc2sys: Fix lower bound value of update_rate

Following command produces unexpected update rate.
 # phc2sys -s eth0 -q -m -O0 -R1.0842021724855044e-19
This is caused by wrong validation of phc_interval related to
significant digits of double precision and phc_interval_tp.tv_sec
overflow.

To avoid these unwanted trouble, this patch limits lower bound of
phc_rate to 1e-9.
There is no profound meaning in the lower bound value. I just think
it's enough to actual use and it doesn't cause phc_interval_tp.tv_sec
overflow in not only 64bit environment but also 32bit environment.
Thereby, messy validation of phc_interval is no longer needed.

Signed-off-by: Ken ICHIKAWA <ichikawa.ken@jp.fujitsu.com>