BoringTun should be Linux network namespace aware

Setup scenario:

1. Create two network namespaces
2. In each namespace create an interface with `boringtun [-f] `

Now I can use the `wg(8)` tool in either namespace (and probably from outside as well) to configure any interface in any network namespace. Moreover, if I use the same interface name in both namespaces, the `wg(8)` tool will show only 1 interface and setting configuration options using `wg(8)` actually overwrites both interface configs.

Expected behaviour (on parity with the kernel implementation):

I can independently create and configure a wireguard interface in each network namespace. The interfaces can even be named in the same way (for example `wg0`) and the `wg(8)` sees and configures only interfaces from its own network namespace.