Fix implicit DeviceMapper selection

DeviceMapper must be explicitly selected because the Docker binary might not be linked to the right devmapper library.

With this change, Docker fails fast if the driver detection finds the devicemapper directory but the driver is not the default option.
The option `override_udev_sync_check` doesn't make sense anymore, since the user must be explicit to select devicemapper, so it's being removed.
Docker fails to use devicemapper only if Docker has been built statically unless the option was explicit.

Signed-off-by: David Calavera <david.calavera@gmail.com>

docs/reference/commandline/daemon.md

@@ -323,42 +323,6 @@ options for `zfs` start with `zfs`.
 
         $ docker -d --storage-opt dm.blkdiscard=false
 
- *  `dm.override_udev_sync_check`
-
-    Overrides the `udev` synchronization checks between `devicemapper` and `udev`.
-    `udev` is the device manager for the Linux kernel.
-
-    To view the `udev` sync support of a Docker daemon that is using the
-    `devicemapper` driver, run:
-
-        $ docker info
-        [...]
-        Udev Sync Supported: true
-        [...]
-
-    When `udev` sync support is `true`, then `devicemapper` and udev can
-    coordinate the activation and deactivation of devices for containers.
-
-    When `udev` sync support is `false`, a race condition occurs between
-    the`devicemapper` and `udev` during create and cleanup. The race condition
-    results in errors and failures. (For information on these failures, see
-    [docker#4036](https://github.com/docker/docker/issues/4036))
-
-    To allow the `docker` daemon to start, regardless of `udev` sync not being
-    supported, set `dm.override_udev_sync_check` to true:
-
-	$ docker -d --storage-opt dm.override_udev_sync_check=true
-
-    When this value is `true`, the  `devicemapper` continues and simply warns
-    you the errors are happening.
-
-    > **Note:**
-    > The ideal is to pursue a `docker` daemon and environment that does
-    > support synchronizing with `udev`. For further discussion on this
-    > topic, see [docker#4036](https://github.com/docker/docker/issues/4036).
-    > Otherwise, set this flag for migrating existing Docker daemons to
-    > a daemon with a supported environment.
-
 
 ## Docker execdriver option
 

man/docker.1.md

@@ -451,45 +451,6 @@ removed.
 
 Example use: `docker -d --storage-opt dm.blkdiscard=false`
 
-#### dm.override_udev_sync_check
-
-By default, the devicemapper backend attempts to synchronize with the
-`udev` device manager for the Linux kernel.  This option allows
-disabling that synchronization, to continue even though the
-configuration may be buggy.
-
-To view the `udev` sync support of a Docker daemon that is using the
-`devicemapper` driver, run:
-
-        $ docker info
-	[...]
-	 Udev Sync Supported: true
-	[...]
-
-When `udev` sync support is `true`, then `devicemapper` and `udev` can
-coordinate the activation and deactivation of devices for containers.
-
-When `udev` sync support is `false`, a race condition occurs between
-the`devicemapper` and `udev` during create and cleanup. The race
-condition results in errors and failures. (For information on these
-failures, see
-[docker#4036](https://github.com/docker/docker/issues/4036))
-
-To allow the `docker` daemon to start, regardless of whether `udev` sync is
-`false`, set `dm.override_udev_sync_check` to true:
-
-        $ docker -d --storage-opt dm.override_udev_sync_check=true
-
-When this value is `true`, the driver continues and simply warns you
-the errors are happening.
-
-**Note**: The ideal is to pursue a `docker` daemon and environment
-that does support synchronizing with `udev`. For further discussion on
-this topic, see
-[docker#4036](https://github.com/docker/docker/issues/4036).
-Otherwise, set this flag for migrating existing Docker daemons to a
-daemon with a supported environment.
-
 # EXEC DRIVER OPTIONS
 
 Use the **--exec-opt** flags to specify options to the exec-driver. The only