semanage: disconnect to free libsemanage internals

Destroying the handle does not actually disconnect, see [1].
Also free the key on user removal.

[1]: https://github.com/SELinuxProject/selinux/blob/e9072e7d45f4559887d11b518099135cbe564163/libsemanage/src/direct_api.c#L330

Example adduser leak:

    Direct leak of 1008 byte(s) in 14 object(s) allocated from:
        #0 0x5638f2e782ae in __interceptor_malloc (./src/useradd+0xee2ae)
        shadow-maint#1 0x7fb5cfffad09 in dbase_file_init src/database_file.c:170:45

    Direct leak of 392 byte(s) in 7 object(s) allocated from:
        #0 0x5638f2e782ae in __interceptor_malloc (./src/useradd+0xee2ae)
        shadow-maint#1 0x7fb5cfffc929 in dbase_policydb_init src/database_policydb.c:187:27

    Direct leak of 144 byte(s) in 2 object(s) allocated from:
        #0 0x5638f2e782ae in __interceptor_malloc (./src/useradd+0xee2ae)
        shadow-maint#1 0x7fb5cfffb519 in dbase_join_init src/database_join.c:249:28

    [...]

lib/semanage.c

@@ -97,6 +97,8 @@ static semanage_handle_t *semanage_init (void)
 	return handle;
 
 fail:
+	if (handle)
+		semanage_disconnect (handle);
 	semanage_handle_destroy (handle);
 	return NULL;
 }
@@ -144,7 +146,7 @@ static int semanage_user_mod (semanage_handle_t *handle,
 
 
 static int semanage_user_add (semanage_handle_t *handle,
-                             semanage_seuser_key_t *key,
+                             const semanage_seuser_key_t *key,
                              const char *login_name,
                              const char *seuser_name)
 {
@@ -255,6 +257,8 @@ int set_seuser (const char *login_name, const char *seuser_name)
 
 done:
 	semanage_seuser_key_free (key);
+	if (handle)
+		semanage_disconnect (handle);
 	semanage_handle_destroy (handle);
 	return ret;
 }
@@ -329,6 +333,9 @@ int del_seuser (const char *login_name)
 
 	ret = 0;
 done:
+	semanage_seuser_key_free (key);
+	if (handle)
+		semanage_disconnect (handle);
 	semanage_handle_destroy (handle);
 	return ret;
 }