general: add port_name to ppi, use it for diagnostics (not iface_name)

Otherwise, if we have two PTP ports for the same interface, like
Ethernet and UDP, we are not able to tell which message is for which.

This now works as expected:

./ppsi -C "port eth0;  iface eth0; role master; proto raw; diagnostics 111" \
       -C "port eth0u; iface eth0; role master; proto udp; diagnostics 111"

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>