fix: setup /bin/login for native systemd support

[psmf]

When native systemd integration is enabled in WSL, it looks for /bin/login to login the user.

It enables proper support for user session (dbus and systemd).

Resolves issue #165.

[psvo]

System processes with /bin/login symlink:

C:\> wsl --shutdown
C:\> wsl

bsh ❯ ps xafu
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  1.0  0.0  17608 10628 ?        Ss   17:28   0:00 /nix/var/nix/profiles/system/systemd/lib/systemd/systemd /sbin/init
root           2  0.0  0.0   2300  1188 ?        Sl   17:28   0:00 /init
root           3  0.0  0.0   2272     4 ?        Sl   17:28   0:00  \_ plan9 --control-socket 6 --log-level 4 --server-fd 7 --pipe-fd 9 --log-truncate
root         541  0.0  0.0   2308   132 ?        Ss   17:28   0:00  \_ /init
root         542  0.0  0.0   2324   140 ?        S    17:28   0:00  |   \_ /init
petr         543  0.1  0.0   9812  6440 pts/0    Ss   17:28   0:00  |       \_ -bash
petr         718  0.0  0.0 227464  3700 pts/0    R+   17:28   0:00  |           \_ ps xafu
root         544  0.0  0.0   5664  3208 pts/1    Ss   17:28   0:00  \_ /bin/login -f
petr         607  0.0  0.0 228124  7236 pts/1    S+   17:28   0:00      \_ -bash
root         165  0.5  0.0  39292 14472 ?        Ss   17:28   0:00 /nix/store/9xmrmm20hhil0yha82zh58wxpfy68gxh-systemd-250.8/lib/systemd/systemd-journald
root         193  1.1  0.4 2041968 77484 ?       Ssl  17:28   0:00 /nix/store/aibd0jvjbyi1rgg43wzsyqfij2ar9kxy-moby-20.10.18/libexec/docker/dockerd --config-file=/nix/store/rr17bi4wvjqplfzyy6arnllxazcys71z-daemon.json
root         245  0.4  0.3 1947788 58420 ?       Ssl  17:28   0:00  \_ containerd --config /var/run/docker/containerd/containerd.toml --log-level info
nscd         194  0.0  0.0 933660  3016 ?        Ssl  17:28   0:00 nscd
root         206  0.1  0.0  15260  7296 ?        Ss   17:28   0:00 /nix/store/9xmrmm20hhil0yha82zh58wxpfy68gxh-systemd-250.8/lib/systemd/systemd-logind
message+     214  0.0  0.0   8528  3836 ?        Ss   17:28   0:00 /nix/store/dqc3aq11bn9z1515mkb87hcsxvvqil47-dbus-1.14.4/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
root         244  0.1  0.1 538148 23472 ?        Ssl  17:28   0:00 nix-daemon --daemon
petr         552  0.1  0.0  16696  9908 ?        Ss   17:28   0:00 /nix/store/9xmrmm20hhil0yha82zh58wxpfy68gxh-systemd-250.8/lib/systemd/systemd --user
petr         555  0.0  0.0  19228  2396 ?        S    17:28   0:00  \_ (sd-pam)
petr         595  0.0  0.0 223740  3360 ?        Ss   17:28   0:00  \_ /nix/store/h3dwyhxyswzr8vkdchiqzdf7s8rlc4if-bash-5.1-p16/bin/bash /nix/store/wm1n86h1lqh605dzpg0j63dfa266nw0z-auto-fix-vscode-server.sh
petr         702  0.0  0.0 223740  1864 ?        S    17:28   0:00      \_ /nix/store/h3dwyhxyswzr8vkdchiqzdf7s8rlc4if-bash-5.1-p16/bin/bash /nix/store/wm1n86h1lqh605dzpg0j63dfa266nw0z-auto-fix-vscode-server.sh
petr         703  0.0  0.0   2788   280 ?        S    17:28   0:00          \_ inotifywait -q -m -e CREATE,ISDIR -e DELETE_SELF --format %w%f:%e /home/petr/.vscode-server/bin

bsh ❯ systemctl --user status
● nixos
    State: running
     Jobs: 0 queued
   Failed: 0 units
    Since: Fri 2022-11-25 17:28:19 CET; 28s ago
   CGroup: /user.slice/user-1000.slice/[email protected]
           ├─app.slice
           │ └─auto-fix-vscode-server.service
           │   ├─ 595 /nix/store/h3dwyhxyswzr8vkdchiqzdf7s8rlc4if-bash-5.1-p16/bin/bash /nix/store/wm1n86h1lqh605dzpg0j63dfa266nw0z-auto-fix-vscode-server.sh
           │   ├─ 702 /nix/store/h3dwyhxyswzr8vkdchiqzdf7s8rlc4if-bash-5.1-p16/bin/bash /nix/store/wm1n86h1lqh605dzpg0j63dfa266nw0z-auto-fix-vscode-server.sh
           │   └─ 703 inotifywait -q -m -e CREATE,ISDIR -e DELETE_SELF --format %w%f:%e /home/petr/.vscode-server/bin
           └─init.scope
             ├─ 552 /nix/store/9xmrmm20hhil0yha82zh58wxpfy68gxh-systemd-250.8/lib/systemd/systemd --user
             └─ 555 "(sd-pam)"

[K900]

r+ from me, tested working on 11 too.