ASCI: Add simple implementation of Group Call Register

This is a built-in data structure to store and handle voice group calls.

The GCR will be used by VGCS/VBS call control.
 (Chg-Id: I9947403fde8212b66758104443c60aaacc8b1e7b)

The GCR will be used by VTY code.
 (Chg-Id: I5bd034a62fc8b483f550d29103c2f7587198f590)

Change-Id: Ia74a4a865f943c5fb388cd28f9406005c92e663e
Related: OS#4854
diff --git a/include/osmocom/msc/asci_gcr.h b/include/osmocom/msc/asci_gcr.h
new file mode 100644
index 0000000..f2160c1
--- /dev/null
+++ b/include/osmocom/msc/asci_gcr.h
@@ -0,0 +1,55 @@
+/* Group Call Register (GCR) */
+/*
+ * (C) 2023 by sysmocom - s.f.m.c. GmbH <info@sysmocom.de>
+ * All Rights Reserved
+ *
+ * SPDX-License-Identifier: AGPL-3.0+
+ *
+ * Author: Andreas Eversberg
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+/* Group Call Register */
+struct gcr {
+	struct llist_head list;
+	enum trans_type trans_type;
+	char group_id[9];
+	uint16_t timeout;
+	bool mute_talker;
+	struct llist_head bss_list;
+};
+
+struct gcr_bss {
+	struct llist_head list;
+	int pc;
+	struct llist_head cell_list;
+};
+
+struct gcr_cell {
+	struct llist_head list;
+	uint16_t cell_id;
+};
+
+struct gcr_cell *gcr_add_cell(struct gcr_bss *bss, uint16_t cell_id);
+struct gcr_cell *gcr_find_cell(struct gcr_bss *bss, uint16_t cell_id);
+void gcr_rm_cell(struct gcr_bss *bss, uint16_t cell_id);
+struct gcr_bss *gcr_add_bss(struct gcr *gcr, int pc);
+struct gcr_bss *gcr_find_bss(struct gcr *gcr, int pc);
+void gcr_rm_bss(struct gcr *gcr, int pc);
+struct gcr *gcr_create(struct gsm_network *gsmnet, enum trans_type trans_type, const char *group_id);
+void gcr_destroy(struct gcr *gcr);
+struct gcr *gcr_by_group_id(struct gsm_network *gsmnet, enum trans_type trans_type, const char *group_id);
+struct gcr *gcr_by_callref(struct gsm_network *gsmnet, enum trans_type trans_type, uint32_t callref);