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/gsubprocess-testprog.c

@@ -23,9 +23,10 @@ write_all (int           fd,
   while (len > 0)
     {
       gssize bytes_written = write (fd, buf, len);
+      int errsv = errno;
       if (bytes_written < 0)
 	g_error ("Failed to write to fd %d: %s",
-		 fd, g_strerror (errno));
+		 fd, g_strerror (errsv));
       buf += bytes_written;
       len -= bytes_written;
     }