Tell pppd not to ask for IPv6 addresses during dial-up

Tell pppd not to ask for IPv6 addresses during dial-up

pppd 2.4.9 supports IPv6 and asks for an IPv6 configuration by default.
Setting the received prefix in the kernel will never work, however, as
the rest of IPFire 2.x does not support IPv6.

pppd notices the ISP about this, and at least Otenet (GR) and British
Telecom (several countries) decide to close a dial-up connection then.
German DTAG seems to ignore such errors silently.

This patch adds an option to the pppd call to prevent asking for an
IPv6 configuration, hence avoiding this errors.

To apply this patch, it is necessary to ship ppp 2.4.9 again. Since I
have no access to a testing machine behind an ISP supporting IPv6, this
patch unfortunately is untested.

Fixes: #12651
Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>