eCAP support, phase 2: Implemented libecap interfaces, added eCAP

squid.conf options. Link with libecap when eCAP support is enabled.

eCAP code needs polishing and enhancement but appears to work for a few
targeted cases. I am committing this now so that users working on eCAP
modules can test and provide more specific feedback.

These adaptation-specific changes should not have significant effect on
core code.

The libecap library is available at http://www.e-cap.org/