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/gcharsetconverter.c

@@ -433,6 +433,7 @@ g_charset_converter_initable_init (GInitable     *initable,
 				   GError       **error)
 {
   GCharsetConverter  *conv;
+  int errsv;
 
   g_return_val_if_fail (G_IS_CHARSET_CONVERTER (initable), FALSE);
 
@@ -446,10 +447,11 @@ g_charset_converter_initable_init (GInitable     *initable,
     }
 
   conv->iconv = g_iconv_open (conv->to, conv->from);
+  errsv = errno;
 
   if (conv->iconv == (GIConv)-1)
     {
-      if (errno == EINVAL)
+      if (errsv == EINVAL)
 	g_set_error (error, G_IO_ERROR, G_IO_ERROR_NOT_SUPPORTED,
 		     _("Conversion from character set “%s” to “%s” is not supported"),
 		     conv->from, conv->to);