polypath: Provide Linux interface monitor

This script monitors the local network interfaces and prints output as
interfaces and addresses appear and disappear.

Install it in $(datarootdir).  Provide $(datarootdir) to C code via a
-D compiler flag.  Provide the appropriate path as the default setting
for `monitor-command' in the polypath module.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>