Change fd[0] with FD_EVENT and fd[1] with FD_GENERAL.

This change ensure that all timers timeouts are handled just before the
PTP message handling.
The PTP message hanlding code can performe timerfd_settime() calls that
invalidate the status of timer descriptors. This side effect is not
noticed until now because we never read from the timer descriptors.

After the timer expiration, just after the poll(), we can read fron the
timer file descriptor a 64bit value that represents the number of
expirations that have occured. This value is useful for debug. In
addition, when we use a periodic timer the timer will be locked until
the read of this value.

During a timer handling we should avoid to call timerfd_settime() on a
different timer because this call will invalidate the file descriptor,
resulting in a EGAIN when the other timer code will performe the read().

The ANNOUNCE and RX SYNC timer code just doesn't follow this rule. The
solution is to split the rearm code.

In order to obtain a precise TX SYNC frequency we need to use a periodic
timer facility instead to reconfigure the timer on every expiration.

The solution consists to configure the it_interval field of the
itimerspec structure (see timerfd_create(2) man page) only when the port
goes into Master role and avoiding to call timerfd_settime() on timer
expiration.