VPNSP value custom for OpenVPN custom config files (#621)

- Retro-compatibility: `OPENVPN_CUSTOM_CONFIG` set implies `VPNSP=custom`
- Change: `up` and `down` options are not filtered out
- Change: `OPENVPN_INTERFACE` overrides the network interface defined in the configuration file
- Change: `PORT` overrides any port found in the configuration file
- Feat: config file is read when building the OpenVPN configuration, so it's effectively reloaded on VPN restarts
- Feat: extract values from custom file at start to log out valid settings
- Maint: `internal/openvpn/extract` package instead of `internal/openvpn/custom` package
- Maint: All providers' `BuildConf` method return an error
- Maint: rename `CustomConfig` to `ConfFile` in Settings structures

maintenance.md

@@ -28,6 +28,7 @@
 - Gluetun entire logs available at control server, maybe in structured format
 - Authentication with the control server
 - Get announcement from Github file
+- Support multiple connections in custom ovpn
 
 ## Gluetun V4
 
@@ -47,4 +48,7 @@
 - Change `VPNSP` to `VPN_SERVICE_PROVIDER`
 - Change `REGION` (etc.) to `SERVER_REGIONS`
 - Remove `PUBLICIP_FILE`
+- Remove retro-compatibility where OPENVPN_CONFIG != "" implies VPNSP = "custom"
+ and set `OPENVPN_CUSTOM_CONFIG` default to `/gluetun/custom.ovpn`
 - Split servers.json and compress it
+- Use relative paths everywhere instead of absolute