Support libecap v1.0, allowing asynchronous adapters and eCAP version checks.

Support libecap v1.0, allowing asynchronous adapters and eCAP version checks.

After these changes, Squid can support eCAP adapters built with libecap v1.0,
but stops supporting adapters built with earlier libecap versions (due to API
changes). The new libecap version allows Squid to better check the version of
the eCAP adapter being loaded as well as the version of the eCAP library being
used. This should help with migration to libecap v1.0.

Expose [running] main event loop as a global so that modules can add engines.