Add a place for a driver to report rx timestamps

in nanoseconds from boot for the received packets.

The rcv_tstmp field overlaps the place of Ln header length indicators,
not used by received packets.  The basic pkthdr rearrangement change
in sys/mbuf.h was provided by gallatin.

There are two accompanying M_ flags: M_TSTMP means that there is the
timestamp (and it was generated by hardware).

Another flag M_TSTMP_HPREC indicates that the timestamp is
high-precision.  Practically M_TSTMP_HPREC means that hardware
provided additional precision comparing with the stamps when the flag
is not set.  E.g., for ConnectX all packets are stamped by hardware
when PCIe transaction to write out the completion descriptor is
performed, but PTP packet are stamped on port.  For Intel cards, when
PTP assist is enabled, only PTP packets are stamped in the limited
number of registers, so if Intel cards ever start support this
mechanism, they would always set M_TSTMP | M_TSTMP_HPREC if hardware
timestamp is present for the given packet.

Add IFCAP_HWRXTSTMP interface capability to indicate the support for
hardware rx timestamping, and ifconfig(8) command to toggle it.

Based on the patch by:	gallatin
Reviewed by:	gallatin (previous version), hselasky
Sponsored by:	Mellanox Technologies
MFC after:	2 weeks (? mbuf KBI issue)
X-Differential revision:	https://reviews.freebsd.org/D12638

newlib/libc/sys/rtems/include/net/if.h

@@ -240,6 +240,7 @@ struct if_data {
 #define	IFCAP_TXCSUM_IPV6	0x400000  /* can offload checksum on IPv6 TX */
 #define	IFCAP_HWSTATS		0x800000 /* manages counters internally */
 #define	IFCAP_TXRTLMT		0x1000000 /* hardware supports TX rate limiting */
+#define	IFCAP_HWRXTSTMP		0x2000000 /* hardware rx timestamping */
 
 #define IFCAP_HWCSUM_IPV6	(IFCAP_RXCSUM_IPV6 | IFCAP_TXCSUM_IPV6)