Teach /etc/rc.d/ppp to start multiple instances of ppp.

ppp_profile variable can now contain multiple profiles.
Overrides for ppp mode and nat can go into ppp_$profile_mode
and ppp_$profile_nat variables respectively. If those are
not specified, defaults from ppp_mode and ppp_nat are used.

Submitted by: Yuri Kurenkov < y dot kurenkov at init dot ru >
Reviewed by: mtm
MFC after: 1 week