Remove snapshot unit type

Snapshots were never useful or used for anything. Many systemd
developers that I spoke to at systemd.conf2015, didn't even know they
existed, so it is fairly safe to assume that this type can be deleted
without harm.

The fundamental problem with snapshots is that the state of the system
is dynamic, devices come and go, users log in and out, timers fire...
and restoring all units to some state from the past would "undo"
those changes, which isn't really possible.

Tested by creating a snapshot, running the new binary, and checking
that the transition did not cause errors, and the snapshot is gone,
and snapshots cannot be created anymore.

New systemctl says:
Unknown operation snapshot.
Old systemctl says:
Failed to create snapshot: Support for snapshots has been removed.

IgnoreOnSnaphost settings are warned about and ignored:
Support for option IgnoreOnSnapshot= has been removed and it is ignored

http://lists.freedesktop.org/archives/systemd-devel/2015-November/034872.html

1 files changed, 0 insertions, 40 deletions

diff --git a/man/systemctl.xml b/man/systemctl.xml
index 2fe24fd139..173c463d12 100644
--- a/man/systemctl.xml
+++ b/man/systemctl.xml
@@ -1342,46 +1342,6 @@ kobject-uevent 1 systemd-udevd-kernel.socket systemd-udevd.service
     </refsect2>
 
     <refsect2>
-      <title>Snapshot Commands</title>
-
-      <variablelist>
-        <varlistentry>
-          <term><command>snapshot <optional><replaceable>NAME</replaceable></optional></command></term>
-
-          <listitem>
-            <para>Create a snapshot. If a snapshot name is specified,
-            the new snapshot will be named after it. If none is
-            specified, an automatic snapshot name is generated. In
-            either case, the snapshot name used is printed to standard
-            output, unless <option>--quiet</option> is specified.
-            </para>
-
-            <para>A snapshot refers to a saved state of the systemd
-            manager. It is implemented itself as a unit that is
-            generated dynamically with this command and has dependencies
-            on all units active at the time. At a later time, the user
-            may return to this state by using the
-            <command>isolate</command> command on the snapshot unit.
-            </para>
-
-            <para>Snapshots are only useful for saving and restoring
-            which units are running or are stopped, they do not
-            save/restore any other state. Snapshots are dynamic and lost
-            on reboot.</para>
-          </listitem>
-        </varlistentry>
-        <varlistentry>
-          <term><command>delete <replaceable>PATTERN</replaceable>...</command></term>
-
-          <listitem>
-            <para>Remove a snapshot previously created with
-            <command>snapshot</command>.</para>
-          </listitem>
-        </varlistentry>
-      </variablelist>
-    </refsect2>
-
-    <refsect2>
       <title>Environment Commands</title>
 
       <variablelist>