Add LDAP support (RFC 4511) to s_client ("-starttls ldap")

Based on initial patch by Alex Bergmann <alex@linlab.net> and new function
ldap_ExtendedResponse_parse() by Andy Polyakov <appro@openssl.org>. Thanks
very much to both.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2293)