Revert "Rationalize DNS options" (#416)

This reverts commit f3830844.

See #349 and #406.

With f3830844 it is not possible anymore to use openfortivpn's internal mechanism to update /etc/resolv.conf, because it also needs to pass the usepeerdns option to pppd in order to receive this information from the vpn server. Here a symmary of the combinations of the options and the result:

New behavior without specific ip-up scripts for the ppp interface or openresolv service:
set-dns 	pppd-use-peerdns 	/var/run/ppp/resolv.conf 	/etc/resolv.conf
true 	true 	created/updated 	not updated (default case)
true 	false 	not created/updated 	not updated (broken)
false 	true 	not created/updated 	not updated
false 	false 	not created/updated 	not updated

old behavior (without specific ip-up scripts for the ppp interface or openresolv):
set-dns 	pppd-use-peerdns 	/var/run/ppp/resolv.conf 	/etc/resolv.conf
true 	true 	created/updated 	updated (default case)
true 	false 	not created/updated 	not updated (expected but not very intuitive)
false 	true 	created/updated 	not updated (intended for openresolv or ip-up script)
false 	false 	not created/updated 	not updated

With ip-up script or openresolv, the new default behavior is the required one, with the old behavior set-dns=0 would be needed.
Without ip-up script or openresolv, the old default behavior is the required one, whereas there is no combination that can update /etc/resolv.conf out of the box.