systemd/enable_overcommit_memory: Use /sbin/sysctl for Ubuntu

Ubuntu’s package *procps* 2:3.3.16-5ubuntu3.1 installs `sysctl` only in
`/sbin/sysctl`, and therefore the service unit fails to be started.

Instead of `/usr/sbin/sysctl`, use `/sbin/sysctl`, which is a symbolic
link on MarIuX.

    $ ls -l /sbin/sysctl # usrlinks-1.0-0.x86_64
    lrwxrwxrwx 1 root root 16 May 29 15:02 /sbin/sysctl -> /usr/sbin/sysctl

Alternatively, as systemd does not require absolute pathnames anymore,
only `sysctl` could be used, but be explicit for now.

misc_systemd_units/enable_overcommit_memory.service

@@ -8,8 +8,8 @@ ConditionHost=|flughafenberlinbrandenburgwillybrandt.molgen.mpg.de
 [Service]
 Type=oneshot
 RemainAfterExit=yes
-ExecStart=/usr/sbin/sysctl vm.overcommit_memory=0
-ExecStop=/usr/sbin/sysctl vm.overcommit_memory=2
+ExecStart=/sbin/sysctl vm.overcommit_memory=0
+ExecStop=/sbin/sysctl vm.overcommit_memory=2
 
 [Install]
 WantedBy=graphical.target