MINOR: tcpcheck: Use tcpcheck flags to know a healthcheck uses SSL connections

MINOR: tcpcheck: Use tcpcheck flags to know a healthcheck uses SSL connections

The proxy flag PR_O_TCPCHK_SSL is replaced by a flag on the tcpcheck
itself. When TCPCHK_FL_USE_SSL flag is set, it means the healthcheck will
use an SSL connection and the SSL xprt must be prepared for the server.