ppp-manager: give PPP more time to terminate

pppd is a delicate flower. On orderly shutdown, it likes to tell the
other side. This seems to take at least a second even when no real
network latency is at play, on busy systems 1.5 seconds easily ends up
being inadequate.

A violent shutdown is generally okay apart from that it can leave
garbage (port lock) behind and the other side potentially confused for a
while.

As it happens, this interacts badly with modemu.pl which is used for
testing: the pseudo terminal in PPP line discipline mode has no idea
that the remote disconnected and while ModemManager is learning that
something wrong the hard way (AT command timing out, because the remote
still expects to talk PPP), the test times out.

Let's increase the timeout to something more reasonable.

https://bugzilla.redhat.com/show_bug.cgi?id=2049596
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