User keys master

[xxxserxxx]

Replaces #77. This PR is based on the head of master and should merge cleanly without any extra changes. It also handles the CLI parameters in a more viper-ish way. In the bargain, it implements your todo code comment about allowing users to provide a public key.

This PR adds the ability to accept user-supplied private and public keys. The use case for public keys is obvious, but there's also a use case for private keys.

The WireGuard Android client does not allow two WireGuard configurations to be active at the same time. The wg-quick config format also only allows declaring one public key. Therefore, if a user wants to have two subnets active (on a mobile phone), then they have to re-use a private key.

The patch adds two (optional) parameters to Add -- --private-key and --public-key.

• If either is provided, dsnet will prompt the user to enter the private and/or public key
• If only a private key is provided, dsnet will generate the public key from the supplied private key
• If only a public key is provided, dsnet will use an empty private key. This will result in a private key of AAAAA...AAAAA=, which the user will have to replace with their private key before using.
• If both keys are provided, dsnet will use them both. It will also validate that the public and private key pairs match, by re-generating the public key.

I can't think of a reason why a user would provide both keys, but rather than disallow it, I have the code test the keys.

Example:

phaethusa ~ % sudo ./dsnet add -r mynode
private key: RGlkIHlvdSB0aGluayBJIHdvdWxkIHBvc3QgYSBrZXk=
owner: Me
Description: My Phone

Do you want to add the above configuration? [y/n] y

[Interface]
Address=10.79.56.6/22
PrivateKey=RGlkIHlvdSB0aGluayBJIHdvdWxkIHBvc3QgYSBrZXk=

[Peer]
PublicKey=U3RvcCB0cnlpbmchIFRoaXMgaXMgY2Vuc29yZWRlZGQ=
PresharedKey=VGhpcyBpcyBhIHNpbGx5IGJpdCBvZiB0ZXh0LiAwMTI=
Endpoint=my.server.net:51820
PersistentKeepalive=0
AllowedIPs=10.79.56.0/22

This feature is backwards compatible; if neither argument is provided, dsnet generates the keys itself.

[naggie]

Thanks @xxxserxxx -- seems like a worthwhile change. I will put it through soon. @botto please take a look, too.

Sorry for the late reply, I've been swamped!

[xxxserxxx]

Rebased from origin/master for a clean merge.

[xxxserxxx]

Note if you try to merge both of my PRs, there'll be conflicts. If you merge one, I'll rebase and update the other to make it merge cleanly.