The contract for custom extension callbacks has changed - all custom extension callbacks are triggered

Whitespace fixes

If multiple TLS extensions are expected but not received, the TLS extension and supplemental data 'generate' callbacks are the only chance for the receive-side to trigger a specific TLS alert during the handshake.

Removed logic which no-op'd TLS extension generate callbacks (as the generate callbacks need to always be called in order to trigger alerts), and updated the serverinfo-specific custom TLS extension callbacks to track which custom TLS extensions were received by the client, where no-ops for 'generate' callbacks are appropriate.

If an application calls the macro SSL_CTX_get_extra_chain_certs
return either the old "shared" extra certificates or those associated
with the current certificate.

This means applications which call SSL_CTX_use_certificate_chain_file
and retrieve the additional chain using SSL_CTX_get_extra_chain_certs
will still work. An application which only wants to check the shared
extra certificates can call the new macro
SSL_CTX_get_extra_chain_certs_only

This allows to process multiple fragmets of maximum fragment size,
as opposite to chopping maximum-sized fragments to multiple smaller
ones. This approach relies on dynamic allocation of larger buffers,
which we trade for performance improvement, for several *times* in
some situations.

If application has more data than maximum fragment, hold to buffer
for whole write, as opposite to per-fragment strategy.

PR#3253

New ctrl sets current certificate based on certain criteria. Currently
two options: set the first valid certificate as current and set the
next valid certificate as current. Using these an application can
iterate over all certificates in an SSL_CTX or SSL structure.

Atom Silvermont. On other CPUs one can observe 1% loss on some
algorithms.

+5% on Atom Silvermont, up to +8% improvement of legacy code.
Harmonize sha1-586.pl and aesni-sha1-x86_86.p with sha1-x86_64.pl.

Remove reference to ERR_TXT_MALLOCED in the error library as that is
only used internally. Indicate that returned error data must not be
freed.
(cherry picked from commit f2d678e6)

PR#3244
(cherry picked from commit 9614d2c6)

Always add a dynamically loaded ENGINE to list. Otherwise it can cause
problems when multiply loaded, especially if it adds new public key methods.
For all current engines we only want a single implementation anyway.

Partial fix for PR#3183.

PR#3178

If available rdrand is used as an additional entropy source for the
PRNG and for additional input in FIPS mode.

change documentation and comments to indicate that we prefer the
standard "DHE" naming scheme everywhere over the older "EDH"