vty: add ctrl section for Control interface bind address

This may seem like overkill for a mere const char * config item, but it makes
the Control interface VTY commands reusable in any main() scope (inspired by
libosmo-abis' VTY config).

Add API functions ctrl_vty_init() and ctrl_vty_get_bind_addr(), in new files
src/ctrl/control_vty.c and include/osmocom/ctrl/control_vty.h, compiled and/or
installed dependent on ENABLE_VTY.

Using these functions allows configuring a static const char* with the VTY
commands

    ctrl
     bind A.B.C.D

which callers shall subsequently use to bind the Control interface to a
specific local interface address, by passing the return value of
ctrl_vty_get_bind_addr() to control_interface_setup().

Add CTRL_NODE to enum node_type, "eating" RESERVED4_NODE to heed that comment
on avoiding ABI changes.
diff --git a/include/osmocom/vty/command.h b/include/osmocom/vty/command.h
index 2078e1b..9a02575 100644
--- a/include/osmocom/vty/command.h
+++ b/include/osmocom/vty/command.h
@@ -83,6 +83,7 @@
 	L_IPA_NODE,		/*!< \brief IPA proxying commands in libosmo-abis. */
 	L_NS_NODE,		/*!< \brief NS node in libosmo-gb. */
 	L_BSSGP_NODE,		/*!< \brief BSSGP node in libosmo-gb. */
+	L_CTRL_NODE,		/*!< \brief Control interface node. */
 
 	/*
 	 * When adding new nodes to the libosmocore project, these nodes can be
@@ -91,7 +92,6 @@
 	RESERVED1_NODE,		/*!< \brief Reserved for later extensions */
 	RESERVED2_NODE,		/*!< \brief Reserved for later extensions */
 	RESERVED3_NODE,		/*!< \brief Reserved for later extensions */
-	RESERVED4_NODE,		/*!< \brief Reserved for later extensions */
 
 	_LAST_OSMOVTY_NODE
 };