Add a new servo option which specifies first step threshold

Current pi servo steps clock without any condition on start.
This patch adds a new servo option "configured_pi_f_offset". The option is similar
to configured_pi_offset but only affects in the first clock update. Therefore,
if this option is set as 0.0, we can prevent clock step on start.
The new servo option can be specified from phc2sys by using -F option.

This feature is usefull when we need to restart phc2sys without system
clock jump. Restarting phc2sys is needed to change its configuration.

changes since v2:
 - manual page fix.
 - also apply max_offset along with max_f_offset in servo step1.
 - add a variable to check if first update is done.

changes since v1:(http://sourceforge.net/mailarchive/message.php?msg_id=31039874)
 - remake as a new servo option.

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