Use hardware timestamps to report packet timestamps for SO_TIMESTAMP

Use hardware timestamps to report packet timestamps for SO_TIMESTAMP
and other similar socket options.

Provide new control message SCM_TIME_INFO to supply information about
timestamp.  Currently it indicates that the timestamp was
hardware-assisted and high-precision, for software timestamps the
message is not returned.  Reserved fields are added to ABI to report
additional info about it, it is expected that raw hardware clock value
might be useful for some applications.

Reviewed by:	gallatin (previous version), hselasky
Sponsored by:	Mellanox Technologies
MFC after:	2 weeks
X-Differential revision:	https://reviews.freebsd.org/D12638