optimize lookup of ssl-{unclean,accurate}-shutdown flags:

- only look through the table once, rather than 2 apr_table_gets()
- case-sensitive and use strcmp() as little as possible
- only lookup once per-connection, as the flags will not change across
  keepalive requests
PR:
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk/modules/ssl@92121 13f79535-47bb-0310-9956-ffa450edef68