Remove errno.h include from _scm.h.

* libguile/_scm.h: Remove errno.h include.
* libguile/bytevectors.c:
* libguile/debug.c:
* libguile/finalizers.c:
* libguile/loader.c:
* libguile/poll.c:
* libguile/strings.c:
* libguile/threads.c:
* libguile/vm.c: Add errno.h include.  Sort includes as needed.

libguile/loader.c

@@ -34,6 +34,7 @@
 #include <assert.h>
 #include <alignof.h>
 #include <byteswap.h>
+#include <errno.h>
 #include <verify.h>
 
 #include <full-read.h>