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.c

@@ -615,18 +615,21 @@ read_all_from_fd (gint fd, gsize *out_len, GError **error)
 
   do
     {
+      int errsv;
+
       num_read = read (fd, buf, sizeof (buf));
+      errsv = errno;
       if (num_read == -1)
         {
-          if (errno == EAGAIN || errno == EWOULDBLOCK)
+          if (errsv == EAGAIN || errsv == EWOULDBLOCK)
             continue;
           g_set_error (error,
                        G_IO_ERROR,
-                       g_io_error_from_errno (errno),
+                       g_io_error_from_errno (errsv),
                        "Failed reading %d bytes into offset %d: %s",
                        (gint) sizeof (buf),
                        (gint) str->len,
-                       g_strerror (errno));
+                       g_strerror (errsv));
           goto error;
         }
       else if (num_read > 0)