(cherry picked from commit cbce8c46)

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

Submitted by: Bryan Drewery
PR: 3075
(cherry picked from commit c256e69d)

(cherry picked from commit b74ce8d9)

(cherry picked from commit a9d14832)

(cherry picked from commit 42386fdb)

PR: 3074
(cherry picked from commit 02450ec6)

(cherry picked from commit 8ee3c7e6)

Improve RSA sing performance by 20-30% by:
- switching from floating-point to integer conditional moves;
- daisy-chaining sqr-sqr-sqr-sqr-sqr-mul sequences;
- using MONTMUL even during powers table setup;
(cherry picked from commit 4ddacd99)

(cherry picked from commit 3b848d34)

(cherry picked from commit 42b9a417)

(cherry picked from commit cd8d7335)

(cherry picked from commit c7f690c2)

(cherry picked from commit 32213d8d)

(cherry picked from commit b4275915)

(cherry picked from commit 1bc0b68d)

(cherry picked from commit cdb6c484)

Fix PSS signature printing: consistently use 0x prefix for hex values for
padding length and trailer fields.
(cherry picked from commit deb24ad53147f5a8dd63416224a5edd7bbc0e74a)

(cherry picked from commit 96940f4f2d0300c033379a87db0ff19e598c6264)

previous cbc128.c commit].

(cherry picked from commit b69437e1)

(cherry picked from commit 4df2280b)

(and pending AVX2 changes).
(cherry picked from commit 504bbcf3)

(cherry picked from commit 988d11b6)