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/tests/gdbus-peer-object-manager.c

@@ -219,8 +219,9 @@ setup (Test *test,
 
   if (socketpair (AF_UNIX, SOCK_STREAM, 0, pair) < 0)
     {
-      g_set_error (&error, G_IO_ERROR, g_io_error_from_errno (errno),
-                   "%s", g_strerror (errno));
+      int errsv = errno;
+      g_set_error (&error, G_IO_ERROR, g_io_error_from_errno (errsv),
+                   "%s", g_strerror (errsv));
       g_assert_no_error (error);
     }