Consistently save errno immediately after the operation setting it

Prevent the situation where errno is set by function A, then function B
is called (which is typically _(), but could be anything else) and it
overwrites errno, then errno is checked by the caller.

errno is a horrific API, and we need to be careful to save its value as
soon as a function call (which might set it) returns. i.e. Follow the
pattern:
  int errsv, ret;
  ret = some_call_which_might_set_errno ();
  errsv = errno;

  if (ret < 0)
    puts (strerror (errsv));

This patch implements that pattern throughout GLib. There might be a few
places in the test code which still use errno directly. They should be
ported as necessary. It doesn’t modify all the call sites like this:
  if (some_call_which_might_set_errno () && errno == ESOMETHING)
since the refactoring involved is probably more harmful than beneficial
there. It does, however, refactor other call sites regardless of whether
they were originally buggy.

https://bugzilla.gnome.org/show_bug.cgi?id=785577

gio/gdbusserver.c

@@ -862,17 +862,20 @@ try_tcp (GDBusServer  *server,
       while (bytes_remaining > 0)
         {
           gssize ret;
+          int errsv;
+
           ret = write (fd, server->nonce + bytes_written, bytes_remaining);
+          errsv = errno;
           if (ret == -1)
             {
-              if (errno == EINTR)
+              if (errsv == EINTR)
                 goto again;
               g_set_error (error,
                            G_IO_ERROR,
-                           g_io_error_from_errno (errno),
+                           g_io_error_from_errno (errsv),
                            _("Error writing nonce file at “%s”: %s"),
                            server->nonce_file,
-                           g_strerror (errno));
+                           g_strerror (errsv));
               goto out;
             }
           bytes_written += ret;