Added client-side Server Name Indication (SNI) support in OpenSSL extension.

Added client-side Server Name Indication (SNI) support in OpenSSL extension.
#
# [DOC]
#
# New SSL context options :
#
# - SNI_enabled : Set to FALSE to disable SNI support (enabled by default)
# - SNI_server_name : If not set, the server name will be guessed from the
# stream URL (e.g. https://example.com/ will use example.com as hostname.),
# else the given name will be used.
#
# SNI is to SSL/TLS what the Host header is for HTTP : it allows multiple
# certificates on the same IP address.
#
# As for HTTP virtual hosts, this should be totaly transparent in most cases.
#
# Context options allows more control, e.g. :
#
# $context = stream_context_create(array(
#   'ssl' => array('SNI_server_name' => 'foo.example.com'),
#   'http' => array('header' => 'Host: foo.example.com'),
# ));
# file_get_contents('https://127.0.0.1/', false, $context);
#
# OpenSSL >= 0.9.8j supports SNI (by default since OpenSSL 0.9.8k).