Update 3GPP TS 08.08 Cause handling

* add Class definitions
* add helper to check for extended bit
* add helper to get Cause's Class
* use enum in gsm0808_cause_name() and gsm0808_create_cipher_reject() to
  avoid confusion between class and cause
* update gsm0808_create_cipher_reject() comments

Change-Id: I31b31dfc22eb4b6b07089e1255246ac458125340
Related: OS#3187
diff --git a/include/osmocom/gsm/gsm0808.h b/include/osmocom/gsm/gsm0808.h
index 652aae9..66f7c27 100644
--- a/include/osmocom/gsm/gsm0808.h
+++ b/include/osmocom/gsm/gsm0808.h
@@ -50,7 +50,7 @@
 struct msgb *gsm0808_create_cipher(const struct gsm0808_encrypt_info *ei,
 				   const uint8_t *cipher_response_mode);
 struct msgb *gsm0808_create_cipher_complete(struct msgb *layer3, uint8_t alg_id);
-struct msgb *gsm0808_create_cipher_reject(uint8_t cause);
+struct msgb *gsm0808_create_cipher_reject(enum gsm0808_cause cause);
 struct msgb *gsm0808_create_classmark_request();
 struct msgb *gsm0808_create_classmark_update(const uint8_t *cm2, uint8_t cm2_len,
 					     const uint8_t *cm3, uint8_t cm3_len);
@@ -178,7 +178,8 @@
 
 const char *gsm0808_bssmap_name(uint8_t msg_type);
 const char *gsm0808_bssap_name(uint8_t msg_type);
-const char *gsm0808_cause_name(uint8_t cause);
+const char *gsm0808_cause_name(enum gsm0808_cause cause);
+const char *gsm0808_cause_class_name(enum gsm0808_cause_class class);
 
 extern const struct value_string gsm0808_lcls_config_names[];
 extern const struct value_string gsm0808_lcls_control_names[];