Starting from TLSv1.1 (as seen since draft-ietf-tls-rfc2246-bis-00),
the "certificate_authorities" field grammar of the CertificateRequest
message was redone to allow no distinguished names.  In TLSv1.3, with
the restructured CertificateRequest message, this can be similarly
done by optionally including the "certificate_authorities" extension.
This allows to avoid sending DNs at all.

In practice, aside from published TLS specifications, all supported
SSL/TLS libraries allow to request client certificates with an empty
DN list for any protocol version.  For instance, when operating in
TLSv1, this results in sending the "certificate_authorities" list as
a zero-length vector, which corresponds to the TLSv1.1 specification.
Such behaviour goes back to SSLeay.

The change relaxes the requirement to specify at least one trusted CA
certificate in the ssl_client_certificate directive, which resulted in
sending DNs of these certificates (closes #142).  Instead, all trusted
CA certificates can be specified now using the ssl_trusted_certificate
directive if needed.  A notable difference that certificates specified
in ssl_trusted_certificate are always loaded remains (see 3648ba7db).

Co-authored-by: Praveen Chaudhary <praveenc@nvidia.com>

We own this memory from the session pool.

Previously handlers were mandatory.  However they are not always needed.
For example, a server configured with ssl_reject_handshake does not need a
handler.  Such servers required a fake handler to pass the check.  Now handler
absence check is moved to runtime.  If handler is missing, the connection is
closed with 500 code.

Passing from udp was not possible for the most part due to preread buffer
restriction.  Passing to udp could occasionally work, but the connection would
still be bound to the original listen rbtree, which prevented it from being
deleted on connection closure.

Previously a cycle in pass configuration resulted in stack overflow.

Previously, it could result when left-shifting signed integer due to implicit
integer promotion, such that the most significant bit appeared on the sign bit.

In practice, though, this results in the same left value as with an explicit
cast, at least on known compilers, such as GCC and Clang.  The reason is that
in_addr_t, which is equivalent to uint32_t and same as "unsigned int" in ILP32
and LP64 data type models, has the same type width as the intermediate after
integer promotion, so there's no side effects such as sign-extension.  This
explains why adding an explicit cast does not change object files in practice.

Found with UndefinedBehaviorSanitizer (shift).

Based on a patch by Piotr Sikora.

While copying ngx_http_variable_value_t structures to geo binary base
in ngx_http_geo_copy_values(), and similarly in the stream module,
uninitialized parts of these structures are copied as well.  These
include the "escape" field and possible holes.  Calculating crc32 of
this data triggers uninitialized memory access.

Found with MemorySanitizer.

Signed-off-by: Piotr Sikora <piotr@aviatrix.com>