mod_ssl: add support for subjectAltName-based host name checking in proxy mode
(PR 54030)

factor out code from ssl_engine_init.c:ssl_check_public_cert()
to ssl_util_ssl.c:SSL_X509_match_name()

introduce new SSLProxyCheckPeerName directive, which should eventually
obsolete SSLProxyCheckPeerCN

ssl_engine_io.c:ssl_io_filter_handshake(): avoid code duplication
when aborting with HTTP_BAD_GATEWAY


Fix warning about discarding 'const' qualifier from pointer

Submitted by: kbrand, sf
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1485667 13f79535-47bb-0310-9956-ffa450edef68