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/gunixoutputstream.c

@@ -341,15 +341,18 @@ g_unix_output_stream_write (GOutputStream  *stream,
 
   while (1)
     {
+      int errsv;
+
       poll_fds[0].revents = poll_fds[1].revents = 0;
       do
-	poll_ret = g_poll (poll_fds, nfds, -1);
-      while (poll_ret == -1 && errno == EINTR);
+        {
+          poll_ret = g_poll (poll_fds, nfds, -1);
+          errsv = errno;
+        }
+      while (poll_ret == -1 && errsv == EINTR);
 
       if (poll_ret == -1)
 	{
-          int errsv = errno;
-
 	  g_set_error (error, G_IO_ERROR,
 		       g_io_error_from_errno (errsv),
 		       _("Error writing to file descriptor: %s"),
@@ -364,10 +367,9 @@ g_unix_output_stream_write (GOutputStream  *stream,
 	continue;
 
       res = write (unix_stream->priv->fd, buffer, count);
+      errsv = errno;
       if (res == -1)
 	{
-          int errsv = errno;
-
 	  if (errsv == EINTR || errsv == EAGAIN)
 	    continue;