pppd: set resolv.conf directory appropiately

/etc/ppp/resolv.conf might not be writable and it's not useful, switch
to standard /etc/resolv.conf
If users don't want to update dns lookup entries they shouldn't use the
"userpeerdns" option.

[Thomas: slightly reword the comment in the code.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>