Add support for PureVPN and bug fixes

Macroscopic changes

• PureVPN support
• Fix usage of plaintext DNS #199
• All server location related variables act like optional filters
• Add FIREWALL_VPN_INPUT_PORTS for Vyprvpn port forwarding
• Openvpn get settings HTTP route
• Get port forwarded HTTP route

Microscopic changes

• Fix fatalOnError function behavior
• Better uniformized server filtering for all VPN providers
• Update PIA IP addresses
• Firewall simplifications
• Only get default route and local subnet once at start