More fixes in close handling: Correctly ignore the error writing the close

packet when that should be done. Allow repeated calls to the close cb like
Stdio.File does. Shut down only after close() and destroy(). Don't zero user
set callbacks in shutdown(). Check for close packet in the input buffer in
is_open. Always report close errors to the close callback, even when they
are triggered from ssl_write_callback.

Other fixes: Handle the case when some data is read before an error occurs
in read(). Let write() finish the handshake if we're in blocking mode.

Added set_(non)blocking_keep_callbacks.

Rev: lib/modules/SSL.pmod/sslfile.pike:1.90