Add basic EARFCN support

Add structure representing group of EARFCNs with common priority,
threshold etc.
Add functions to populate this structure.
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 0aeefbb..a6908a2 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -29,6 +29,10 @@
 comp128;
 dbm2rxlev;
 
+osmo_earfcn_add;
+osmo_earfcn_del;
+osmo_earfcn_init;
+
 gprs_cipher_gen_input_i;
 gprs_cipher_gen_input_ui;
 gprs_cipher_load;