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/dbus-launch.c

@@ -39,6 +39,7 @@ write_all (const void *ptr,
   while (len > 0)
     {
       ssize_t done = write (STDOUT_FILENO, p, len);
+      int errsv = errno;
 
       if (done == 0)
         {
@@ -46,10 +47,10 @@ write_all (const void *ptr,
         }
       else if (done < 0)
         {
-          if (errno == EINTR)
+          if (errsv == EINTR)
             continue;
 
-          g_error ("%s: write: %s", ME, g_strerror (errno));
+          g_error ("%s: write: %s", ME, g_strerror (errsv));
         }
       else
         {