Change conn_rec->keepalive to an enumerated value of

Change conn_rec->keepalive to an enumerated value of

AP_CONN_UNKNOWN
AP_CONN_CLOSE
AP_CONN_KEEPALIVE

This also fixes a problem where ap_discard_request_body would not discard
the body when keepalive was 0.  This actually meant the keepalive status
was unknown *not* closed, but no one ever remembered that.

This problem was seen with mod_dav sending error responses (as reported by
Karl Fogel).

Suggested by:	Greg "this isn't the '80s" Stein
Reviewed by:	Greg Ames

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95891 13f79535-47bb-0310-9956-ffa450edef68