Apply utc offset correction even when free running.

When using software time stamping and a free running clock, the
statistics appear to be off by the TAI offset. This patch fixes the
issue by setting the internal UTC timescale flag in such cases.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
ptp4l: Use poll() instead of a try-again loop

This patch modifies sk_receive in order to use poll() on POLLERR instead of the
tryagain loop as this resolves issues with drivers who do not return timestamps
quickly enough. It also resolves the issue of wasting time repeating every
microsecond. It lets the kernel sleep our application until the data or timeout arrives.

This change also replaces the old tx_timestamp_retries config value with
tx_timestamp_timeout specified in milliseconds (the smallest length of time poll
accepts). This does have the side effect of increasing the minimum delay before
missing a timestamp by up to 1ms, but the poll should return sooner in the
normal case where a packet timestamp was not dropped.

This change vastly improves some devices and cleans the code up by simplifying a
race condition window due to drivers returning tx timestamp on the error queue.

[ RC - removed the unused 'try_again' variable. ]

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>