net-snmp: unshare file descriptors namespace

Use unshare(CLONE_FILES) to create a local file descriptors namespace
for the SNMP thread. This is similar to what was done in bf5340
'net-snmp: run snmp in "special" thread', but without calling clone()
directly. So the net-snmp plugin keeps running in a regular pthread.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>

accel-pppd/extra/net-snmp/agent.c

+#include <errno.h>
 #include <pthread.h>
+#include <sched.h>
 #include <signal.h>
+#include <string.h>
 
 #include <net-snmp/net-snmp-config.h>
 #include <net-snmp/net-snmp-includes.h>
@@ -80,6 +83,14 @@ static void *snmp_thread(void *a)
 	sigdelset(&set, 32);
 	pthread_sigmask(SIG_BLOCK, &set, NULL);
 
+	if (unshare(CLONE_FILES) < 0) {
+		log_error("net-snmp: impossible to start SNMP thread:"
+			  " unshare(CLONE_FILES) failed (%s)\n",
+			  strerror(errno));
+
+		return NULL;
+	}
+
 	snmp_register_callback(SNMP_CALLBACK_LIBRARY, SNMP_CALLBACK_LOGGING, agent_log, NULL);
   snmp_disable_log();
 	snmp_enable_calllog();