testptp: add option to enable external timestamping edges

Some drivers (e.g. ice) don't enable any edges by default when external
timestamping is requested by the PTP_EXTTS_REQUEST ioctl, which makes
testptp -e unusable for testing hardware supported by these drivers.

Add -E option to specify if the rising, falling, or both edges should
be enabled by the ioctl.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>