missing: add SIOCGHWTSTAMP to missing.h

In order to allow hwtstamp_ctl to use the non-destructive SIOCGHWTSTAMP
ioctl, we need to add it to missing.h, in order to prevent build failure
on older kernels which don't have this support.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
hwtstamp_ctl: use SIOCGHWTSTAMP ioctl before destructively setting policy

This patch modifies the hwtstamp_ctl program, so that it will (attempt
to) use the SIOCGHWTSTAMP ioctl to non-destructively read the current
hardware timestamping policy, prior to setting it with SIOCSHWTSTAMP.

This change has 3 primary advantages:

1) It allows reading the current settings of the device, which was
   previously not possible since SIOCSHWTSTAMP is destructive.
2) The default behavior without rx-filter or tx-type selected on the
   command line is no longer destructive, since it does not attempt to
   set the values to 0. The user must explicitly request to disable the
   settings, by using the provided options.
3) It allows only modifying tx-type or rx-filter separately, without
   destroying the other setting.

This patch supersedes a previous submission which added a -g flag. This
new method of getting first is more advantageous and doesn't require
adding an additional option flag.

- v4
* only display results if command succeeds, as the contents are otherwise
  expected to be identical to what we passed in.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Bump the date on the hwstamp_ctl man page.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>