Make ap_set_keepalive more statefully aware, allowing it

Make ap_set_keepalive more statefully aware, allowing it
to be called multiple times (to correctly set keepalive)
but not increment keepalives when not needed. This allows
us to handle a special case where we need to discard
body content "early"

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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@104871 13f79535-47bb-0310-9956-ffa450edef68