[layer23/mobile] Improvement and fixes of idle mode process

This patch introduces cell re-relection. When camping on a cell, it
scanns neighbour cells. If a 'better' cell is found, the cell is selected.
If the cell is in a different location area, a location upating is
performed under certain conditions.

The 'better' cell depends on various informations that are broadcasted on
the BCCH of a neihbour cell and of course the RX level. Most operators
don't set these informations, so the 'better' cell depend on a better
RX level for the same location area, or a much better RX level (6 dBm)
at a different location area.

There were many issues at the idle mode process that has been fixed.
Expecially when moving, the state machines got stuck, so no more cell search
was possible, or no further calls / location updating was possible.

In order to see the process of cell selection, enter the VTY interface and
enable the network monitor:

enable
monitor network 1  (where '1' is the instance of the MS)

In order to see the current state of the processes, enter:

show ms

1 files changed, 3 insertions, 0 deletions

diff --git a/src/host/layer23/src/mobile/settings.c b/src/host/layer23/src/mobile/settings.c
index 15397fa6..592f8a8e 100644
--- a/src/host/layer23/src/mobile/settings.c
+++ b/src/host/layer23/src/mobile/settings.c
@@ -39,6 +39,9 @@ int gsm_settings_init(struct osmocom_ms *ms)
 	strcpy(set->layer2_socket_path, layer2_socket_path);
 	strcpy(set->sap_socket_path, sap_socket_path);
 
+	/* network search */
+	set->plmn_mode = PLMN_MODE_AUTO;
+
 	/* IMEI */
 	sprintf(set->imei,   "000000000000000");
 	sprintf(set->imeisv, "0000000000000000");