units: use `systemctl exit` to kill the user manager (#8648)

Use `systemctl --user --force exit` to implement the systemd-exit
user service.

This removes our dependence on an external `kill` binary and the
concerns about whether they recognize SIGRTMIN+n by name or what their
interpretation of SIGRTMIN is.

Tested: `systemctl --user start systemd-exit.service` kills the
  `systemd --user` instance for my user.

meson.build

@@ -563,7 +563,6 @@ splash_bmp = files('test/splash.bmp')
 # /usr/sbin, /sbin, and fall back to the default from middle column.
 progs = [['quotaon',    '/usr/sbin/quotaon'    ],
          ['quotacheck', '/usr/sbin/quotacheck' ],
-         ['kill',       '/usr/bin/kill'        ],
          ['kmod',       '/usr/bin/kmod'        ],
          ['kexec',      '/usr/sbin/kexec'      ],
          ['sulogin',    '/usr/sbin/sulogin'    ],

units/user/systemd-exit.service.in

@@ -16,4 +16,4 @@ After=shutdown.target
 
 [Service]
 Type=oneshot
-ExecStart=@KILL@ -s 58 $MANAGERPID
+ExecStart=@SYSTEMCTL@ --user --force exit