net-snmp: run snmp in "special" thread

NET-SNMP is built on top of stale file descriptor multiplexing mechanism (select)
which prevents to use descriptors with number >= 1024.
Due to accel-ppp uses a lots of descriptors NET-SNMP easy overflows this limit and fails
and even may crash whole accel-ppp process.
This patch runs snmp in special thread which does not share file descritor table
with main process, so it can use less descriptors and live in __FD_SETSIZE limit.
Signed-off-by: NDmitry Kozlov <xeb@mail.ru>

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

 #include <pthread.h>
 #include <signal.h>
+#include <sys/wait.h>
 
 #include <net-snmp/net-snmp-config.h>
 #include <net-snmp/net-snmp-includes.h>
@@ -26,8 +27,8 @@ static int conf_master = 0;
 static oid* oid_prefix;
 static size_t oid_prefix_size;*/
 
-static pthread_t snmp_thr;
 static int snmp_term = 0;
+static int snmp_pid;
 
 /*int accel_ppp_alloc_oid(oid tail, size_t size, oid **oid)
 {
@@ -70,7 +71,7 @@ static int agent_log(int major, int minor, void *serv_arg, void *cl_arg)
 	return 0;
 }
 
-static void *snmp_thread(void *a)
+static int snmp_thread(void *a)
 {
 	sigset_t set;
 
@@ -107,22 +108,22 @@ static void *snmp_thread(void *a)
 	if (conf_master)
 		init_master_agent();
 	
-	while (!snmp_term) {
+	while (!snmp_term)
     agent_check_and_process(1);
-	}
 	
 	snmp_shutdown(conf_agent_name);
 
   SOCK_CLEANUP;
 
-	return NULL;
+	return 0;
 }
 
 static void snmp_ctx_close(struct triton_context_t *ctx)
 {
+	int status;
 	snmp_term = 1;
-	pthread_cancel(snmp_thr);
-	pthread_join(snmp_thr, NULL);
+	kill(snmp_pid, 32);
+	waitpid(snmp_pid, &status, 0);
 	triton_context_unregister(ctx);
 }
 
@@ -146,7 +147,8 @@ static void init(void)
 	if (opt)
 		conf_oid_prefix = opt;*/
 	
-	pthread_create(&snmp_thr, NULL, snmp_thread, NULL);
+	snmp_pid = clone(snmp_thread, malloc(1024*1024) + 1024*1024, CLONE_SIGHAND|CLONE_FS|CLONE_VM|CLONE_THREAD, NULL);
+	
 	triton_context_register(&ctx, NULL);
 	triton_context_wakeup(&ctx);
 	triton_collect_cpu_usage();