Nettle.BufferedCipher: Fixed bug in unpad().

The new unpad() padding checker didn't survive the case where
the padding consisted of a single byte.
SSL.File: Move read_callback setting to read().

The stream read_callback is now set (to ssl_read_callback) whenever
the read buffer isn't full, and cleared when it is.

This reduces the complexity and cleans up the semantics of where
and when the stream read_callback is changed.

Also makes similar changes to the handling of the stream close_callback.

This means that the function update_internal_state() is now a noop.
Strengtened the return type of glob()
Moved pike_get_interpreter_pointer to threads.c
Removed very redundant size_shift debug test
Fixed changelog URL in CHANGES