allow a cleanup pid with replace_cookie_session

Add yaws_api:replace_cookie_session/3 and
yaws_session_server:replace_session/3 to allow the user to pass a cleanup
pid in order to associate a new cleanup process with the replacement
session. Thanks to Patrick Moy for suggesting this.
update priv/mime.types from Apache
scripts/Install: Create ${etcdir}/rc.d on FreeBSD

If Yaws is installed in a non-standard directory, we must create
${etcdir}/rc.d before copying the init script there.
Merge pull request #174 from yakaz/install-freebsd

scripts/Install: Create ${etcdir}/rc.d on FreeBSD