Adds an option to pppd to control whether to replace existing default routes
when using the 'defaultroute' option.
If defaultroute and replacedefaultroute are both set, pppd replaces an existing
default route with the new default route. The old default route is restored when
the connection is taken down.
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org> Co-authored-by: Chris Boot <bootc@debian.org>