Set DTX in Cell Options

* rename field of struct gsm48_cell_options to better match the spec
* add comments with spec references
* add function for setting DTX in cell options struct
* add necessary enum type

Change-Id: I5a8924f57669c951b2e51b663d95f1d360062a54
Reviewed-on: https://gerrit.osmocom.org/39
Reviewed-by: Harald Welte <laforge@gnumonks.org>
Tested-by: Jenkins Builder
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index e0d9dcb..834e5d6 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -162,6 +162,8 @@
 gsm48_number_of_paging_subchannels;
 gsm48_parse_ra;
 gsm48_rr_att_tlvdef;
+gsm48_set_dtx;
+gsm48_dtx_mode;
 gsm48_mi_type_name;
 gsm48_mcc_mnc_to_bcd;
 gsm48_mcc_mnc_from_bcd;