Revert "Constify ctrl_cmd struct fields where appropriate"

This reverts commit ed9d6da.

The commit is good as such, but it causes many compiler warnings in the OpenBSC
build. We want this to be re-applied as soon as we have patches ready that fix
the fallout in openbsc.git.

See also https://lists.osmocom.org/pipermail/openbsc/2016-October/009802.html

Related: OS#1829
Change-Id: I722ad60232a6ef5b4cb984b92c42851de26b3ccd

Author: neeels

TODO-RELEASE

@@ -4,4 +4,3 @@ libosmocore	change major	external talloc dependency / internal talloc removal
 libosmocore	change major	size of ph_data_param struct changed / Extend L1SAP PH-DATA with presence information
 libosmocore	change major	size of ph_data_param struct changed / Extend L1SAP PH-DATA with measurement information
 libosmocore	change major	size of ph_tch_param struct changed / Extend with RTP Marker
-libosmocore	change minor	mark as const id, variable & value fields of ctrl_cmd struct

include/osmocom/ctrl/control_cmd.h

@@ -53,10 +53,10 @@ struct ctrl_connection {
 struct ctrl_cmd {
 	struct ctrl_connection *ccon;
 	enum ctrl_type type;
-	const char *id;
+	char *id;
 	void *node;
-	const char *variable;
-	const char *value;
+	char *variable;
+	char *value;
 	char *reply;
 };