Fix bug 5762: detect missing accept4 that gives ENOSYS

We had been checking for EINVAL, but that means that SOCK_* isn't
supported, not that the syscall itself is missing.

Bugfix on 0.2.3.1-alpha, which started to use accept4.

changes/bug5762

@@ -0,0 +1,4 @@
+  o Minor bugfixes:
+    - Work correctly on Linux systems with accept4 support advertised in
+      their headers, but without accept4 support in the kernel. Fix
+      by murb. Fixes bug 5762; bugfix on 0.2.3.1-alpha.

src/common/compat.c

@@ -1013,10 +1013,11 @@ tor_accept_socket(tor_socket_t sockfd, struct sockaddr *addr, socklen_t *len)
   s = accept4(sockfd, addr, len, SOCK_CLOEXEC);
   if (SOCKET_OK(s))
     goto socket_ok;
-  /* If we got an error, see if it is EINVAL. EINVAL might indicate that,
+  /* If we got an error, see if it is ENOSYS. ENOSYS indicates that,
    * event though we were built on a system with accept4 support, we
-   * are running on one without. */
-  if (errno != EINVAL)
+   * are running on one without. Also, check for EINVAL, which indicates that
+   * we are missing SOCK_CLOEXEC support. */
+  if (errno != EINVAL && errno != ENOSYS)
     return s;
 #endif