ptp4l: add support for using configured_pi_f_offset servo option

This patch adds support for using the configured_pi_f_offset servo option to ptp4l.
If "pi_f_offset_const 0.0" is specified in the config file, stepping on the first
update is prevented. If any other positive value is specified, stepping on the
first update occurs when the offset is larger than the specified value.

change since v1
 - add the new option to default.cfg and gPTP.cfg

Signed-off-by: Ken ICHIKAWA <ichikawa.ken@jp.fujitsu.com>