Rather than using a special mwan3 user to manage mwan3track's tracking
packets, this commit implements a small helper library to bind to
device and to set a fwmark so that the tracking packets can be routed
out of the correct interface.

This provides a consistent method for binding to a device rather than
relying on various packages potentially buggy implementations. For
example: #8139 and #12836

This helper issue also allows for more tracking methods to be added
even if they do not have a command line option to bind to device,
such as iperf3 (eg  #13050).

Signed-off-by: Aaron Goodman <aaronjg@stanford.edu>