[PR-586] Allow setting outgoing IP per user

[justjanne]

In Short

• Adds a database column to the quasseluser table for per-user outgoing IPs
• Binds to that IP when creating new outgoing sockets

Impact

Criteria	Rank	Reason
Impact	★☆☆ 1/3	Allows separating outgoing users by IP, useful as IPv6 identd alternative
Risk	★★★ 3/3	Changes network socket creation, might cause IRC connectivity to fail
Intrusiveness	★☆☆ 1/3	Changes localized to a few classes

Rationale

Currently, Quassel supports identd for per-user ident to allow owners of larger quassel cores to avoid a ban of one user affecting the others. Many networks have stopped supporting this, though, instead opting to move to IPv6 based alternatives.

With this change, the operator of a Quassel Core can set an IPv6 for each individual user, which will be used for outgoing IRC connections, allowing bans against one user to avoid affecting other users.

[justjanne]

Due to the risk involved with this change, I’d like to subject it to significant testing before we merge it and roll it out.

[justjanne]

To use this functionality, you’ll first have to assign the corresponding IPs to an interface.

For example, you can assign a whole block with

ip route add local 2a01:4f8:241:4712::/64 dev enp40s0

[phuzion]

When merged, this may at least partially resolve bug 125.

Perhaps a way to make this "per network" as requested in the bug would be to bind the IP address to an identity, not to a user? Not sure if that adds any complexity to the logic of this PR, but I figured I'd throw out the idea.