stat: Add global stat group

Add a global stat_item group for measurement values and a
corresponding macro to get and set the values.
Add a stat_item STAT_MS_PRESET to monitor the number of
MS objects in the storage.

Sponsored-by: On-Waves ehf
diff --git a/src/gprs_ms_storage.cpp b/src/gprs_ms_storage.cpp
index 36f9b6b..e0aee5e 100644
--- a/src/gprs_ms_storage.cpp
+++ b/src/gprs_ms_storage.cpp
@@ -22,6 +22,7 @@
 #include "gprs_ms_storage.h"
 
 #include "tbf.h"
+#include "bts.h"
 #include "gprs_debug.h"
 
 #define GPRS_UNDEFINED_IMSI "000"
@@ -45,6 +46,8 @@
 void GprsMsStorage::ms_idle(class GprsMs *ms)
 {
 	llist_del(&ms->list());
+	if (m_bts)
+		m_bts->ms_present(m_bts->ms_present_get() - 1);
 	if (ms->is_idle())
 		delete ms;
 }
@@ -90,6 +93,8 @@
 
 	ms->set_callback(this);
 	llist_add(&ms->list(), &m_list);
+	if (m_bts)
+		m_bts->ms_present(m_bts->ms_present_get() + 1);
 
 	return ms;
 }