liblldp_clif add command to return PID of lldpad

When lldpad is invoked with command line option -p, no
/var/run/lldpad.pid file is created. External tools which
need to know the pid of lldpad, need to connect to the
control interface of lldpad and send a ping command
to retrieve the pid.

This patch adds command line option -p to the lldptool to
display the pid of lldpad. The leading PPONG string is
stripped.
Example:
24112

Same behavior for interactive mode. Each ping command
returns the PID of lldpad.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
liblldp_clif add function to return PID of lldpad

Offer a function clif_getpid() to return the pid of
lldpad. Export this function in the liblldp_clif.{a,so}
library for other programs to use it.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
From: Thomas Richter <tmricht@linux.vnet.ibm.com>

vdptest use function clif_getpid

Use new function clif_getpid() to get the pid of lldpad.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>