Added eCAP-specific squid.conf options: ecap_enable and ecap_service.

Added eCAP-specific squid.conf options: ecap_enable and ecap_service.
It is not clear whether we should have ecap_service and icap_service or
merge the two as adaptation_service and just look at the protocol field
of the service URI. One reason to keep the options separate is that some
ICAP services might use http schemes in their URIs, but I do not know how
common that is. Another reason is it makes it easier to add protocol-specific
options.

TODO: Discuss whether similar eCAP and ICAP options should be documented
separately. The alternative is to document one of them and point to that
documentation from the other.

Removed ICAPConfig from deprecated icap_ options that do not use it anymore.