audit: audit calls should return ENODATA when process are not in an audit session

ENODATA is how we usually indicate such "missing info" cases, so we
should do this here, too.

src/basic/audit.c

@@ -36,6 +36,11 @@ int audit_session_from_pid(pid_t pid, uint32_t *id) {
 
         assert(id);
 
+        /* We don't convert ENOENT to ESRCH here, since we can't
+         * really distuingish between "audit is not available in the
+         * kernel" and "the process does not exist", both which will
+         * result in ENOENT. */
+
         p = procfs_file_alloca(pid, "sessionid");
 
         r = read_one_line_file(p, &s);
@@ -47,7 +52,7 @@ int audit_session_from_pid(pid_t pid, uint32_t *id) {
                 return r;
 
         if (u == AUDIT_SESSION_INVALID || u <= 0)
-                return -ENXIO;
+                return -ENODATA;
 
         *id = u;
         return 0;
@@ -68,6 +73,8 @@ int audit_loginuid_from_pid(pid_t pid, uid_t *uid) {
                 return r;
 
         r = parse_uid(s, &u);
+        if (r == -ENXIO) /* the UID was -1 */
+                return -ENODATA;
         if (r < 0)
                 return r;