* fix PKey.check for some broken keys

RSA_check_key is documented to return 1 for valid keys.
It (currently) returns 0 or -1 for invalid ones.
The previous code accepted invalid keys if RSA_check_key returns -1!

* add test

* ALPN: complete handshake without accepting a client's protocols.

The callback passed to `SSL_CTX_set_alpn_select_cb` can return
`SSL_TLSEXT_ERR_NOACK` to allow the handshake to continue without
accepting any of the client's offered protocols.

This commit introduces `NO_OVERLAPPING_PROTOCOLS`, which the Python
callback passed to `Context.set_alpn_select_callback` can return to
achieve the same thing.

It does not change the previous meaning of an empty string, which
still terminates the handshake.

* Update src/OpenSSL/SSL.py

Co-Authored-By: Alex Gaynor <alex.gaynor@gmail.com>

* Address @alex's review.

* Use recorded value in test, fix lint error.

* Cover TypeError branch in _ALPNHelper.callback

* use _ffi.from_buffer(buf) in send, to support bytearray

* add bytearray test

* update CHANGELOG.rst

* move from_buffer before 'buffer too long' check

* context-managed from_buffer + black

* don't shadow buf in send()

* test return count for sendall

* test sending an array

* fix test

* also use from_buffer in bio_write

* de-format _util.py

* formatting

* add simple bio_write tests

* wrap line

* Fix for Python 4

* Fix for Python 4