ipc: change default socket path to ~/.i3/ipc.sock, enable in default config

ipc: change default socket path to ~/.i3/ipc.sock, enable in default config

Having the IPC socket is handy, so enable it by default. Also, if
you have > 1 user running i3 on the same machine, the old path of
/tmp/i3-ipc.sock is not so useful. On the other hand, we needed
quite a bit of changes to make tilde expansion and creation of
directories (mkdir -p) work…