fixing VPN server problem with multiple network interface

[mrpalide]

Did you run make format && make check? Yes

Fixes #1145

Changes:

• add new argument to vpn-server -netifc
• add cli related command

How to test this PR:

• check you have more than one default network interface by ip r command, if not make it.
• make build
• start vpn-server from UI and check logs, you should see an error like this:
  [2022-04-19T04:45:50+04:30] INFO (STDERR) [proc:vpn-server:8ba78bc35b91411081c546dbb5ca1edd]: time="2022-04-19T04:45:50+04:30" level=fatal msg="Error creating VPN server" error="multiple default network interface detected, please set once in setting or be single: [enp0s20u3 wlp7s0]"
• set -netifc argument in vpn-server part of config
• start vpn-server again, and would start normaly
• actually, you can set it by skywire-cli config update vpns --netifc xxx [Not work now, get an error FATAL []: Failed to parse config. error="open : no such file or directory", need handle by @0pcom help ]
• and, you can set it by hypervisor UI too [need Add a netifc field in the UI for configuring the VPN server #1158]

[jdknives]

"multiple default network interfaces detected...set a default one for VPN server or remove one: %v"

[jdknives]

Maybe

ifcs, hasMultiple := s.hasMutipleNetworkInterfaces(...)

[mrpalide]

Done.

[jdknives]

NetworkInterface

[mrpalide]

Done.

[jdknives]

Haven't tested it myself, but if it works, its almost good to go.

[ersonp]

We need to add a check for the value of -netifc and it should be equal to the available interfaces. Currently we can pass anything it it for example

			{
				"name": "vpn-server",
				"args": [
					"-netifc",
					"asdasdasd"
				],
				"auto_start": false,
				"port": 44
			}

and it works.

[2022-04-22T13:52:20+05:30] INFO (STDERR) [proc:vpn-server:5937f58c91eb4e59a409c757314d6b30]: time="2022-04-22T13:52:20+05:30" level=info msg="Got default network interface: asdasdasd"
[2022-04-22T13:52:20+05:30] INFO (STDERR) [proc:vpn-server:5937f58c91eb4e59a409c757314d6b30]: time="2022-04-22T13:52:20+05:30" level=info msg="Got IPs of interface asdasdasd: []"

[ersonp]

We should also add this error

[2022-04-22T14:08:04+05:30] INFO (STDERR) [proc:vpn-server:9a887a337a1148edb8292ef4081421f1]: time="2022-04-22T14:08:04+05:30" level=fatal msg="Error creating VPN server" error="multiple default network interfaces detected...set a default one for VPN server or remove one: [enp4s0 wlp5s0]

to proc errors just like the vpn-client adds it's errors.

[mrpalide]

We should also add this error

[2022-04-22T14:08:04+05:30] INFO (STDERR) [proc:vpn-server:9a887a337a1148edb8292ef4081421f1]: time="2022-04-22T14:08:04+05:30" level=fatal msg="Error creating VPN server" error="multiple default network interfaces detected...set a default one for VPN server or remove one: [enp4s0 wlp5s0]

to proc errors just like the vpn-client adds it's errors.

Because mostly usage of vpn-server is on instances without GUI, I think this feature is not urgent for this release, so I make ticket for this [#1163].