Add support for MPLS on Linux

Closes #135

Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>

doc/interfaces-mpls.scd

@@ -0,0 +1,40 @@
+interfaces-mpls(5)
+
+# NAME
+
+*interfaces-mpls* - MPLS vocabulary for the interfaces(5) file format
+
+# DESCRIPTION
+
+Linux allows has support for MultiProtocol Label Switching (MPLS) for a while
+now.  The following options allow for this configuration.
+
+# MPLS-RELATED OPTIONS
+
+The MPLS executor will only modify the sysctl configuration if these options
+are provided, otherwise other mechanisms such as /etc/sysctl.conf may be used.
+If MPLS is enabled on (at least) one interface the executor will load the
+_mpls_iptunnel_ kernel module.
+
+Be aware that you have to set the _platform_labels_ sysctl to make MPLS work.
+See https://www.kernel.org/doc/Documentation/networking/mpls-sysctl.rst for
+more details on the MPLS related knobs in the Linux kernel.
+
+
+*mpls-enable* _yes|no_
+	Control whether packets can be input on this interface.  If disabled,
+	packets carrying an MPLS label will be discarded without further
+	processing.
+
+# EXAMPLES
+
+```
+iface eth0
+	address 2001:db8:08:15::42/64
+	#
+	mpls-enable yes
+```
+
+# AUTHORS
+
+Maximilian Wilhelm <max@sdn.clinic>

doc/interfaces.scd

@@ -240,6 +240,7 @@ iface eth0
 *interfaces-bond*(5)
 *interfaces-bridge*(5)
 *interfaces-forward*(5)
+*interfaces-mpls*(5)
 *interfaces-ppp*(5)
 *interfaces-vrf*(5)
 *interfaces-vxlan*(5)