Trying cherrypick:

Add support for arbitrary TLS extensions.

Contributed by Trevor Perrin.

Conflicts:

	CHANGES
	ssl/ssl.h
	ssl/ssltest.c
	test/testssl

Fix compilation due to #endif.

Cherrypicking more stuff.

Cleanup of custom extension stuff.

serverinfo rejects non-empty extensions.

Omit extension if no relevant serverinfo data.

Improve error-handling in serverinfo callback.

Cosmetic cleanups.

s_client documentation.

s_server documentation.

SSL_CTX_serverinfo documentation.

Cleaup -1 and NULL callback handling for custom extensions, add tests.

Cleanup ssl_rsa.c serverinfo code.

Whitespace cleanup.

Improve comments in ssl.h for serverinfo.

Whitespace.

Cosmetic cleanup.

Reject non-zero-len serverinfo extensions.

Whitespace.

Make it build.

Conflicts:

	test/testssl