add gsm0808_cell_id_from_cgi(), gsm0808_cell_id_to_cgi()

CGI to Cell ID: for example, for Paging, osmo-msc has a CGI for a subscriber
and needs to send out a Cell Identifier IE. Makes sense to add this conversion
here.

Cell ID to CGI: for a Layer 3 Complete, a subscriber sends the current cell in
the form of a Cell Identifier, which we store as a CGI, if necessary enriched
with the local PLMN.

Add enum with bitmask values to identify parts of a CGI, for the return value
of gsm0808_cell_id_to_cgi(). Can't use enum CELL_IDENT for that, because it
doesn't have a value for just a PLMN (and is not a bitmask).

Change-Id: Ib9af67b100c4583342a2103669732dab2e577b04
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 2d47d7a..3fadc5a 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -205,6 +205,8 @@
 gsm0808_dec_cell_id_list2;
 gsm0808_cell_id_list_add;
 gsm0808_cell_id_to_list;
+gsm0808_cell_id_to_cgi;
+gsm0808_cell_id_from_cgi;
 gsm0808_enc_cell_id;
 gsm0808_dec_cell_id;
 gsm0808_cell_id_name;