Restore errno in %err_detail for ERR_CONNECT_FAIL (#1368)

Restore errno in %err_detail for ERR_CONNECT_FAIL (#1368)

Squid was sometimes logging %err_code/%err_detail as
ERR_CONNECT_FAIL/WITH_SERVER. It now logs
ERR_CONNECT_FAIL/WITH_SERVER+errno=111 (or similar).

When dealing with two error details, Squid was ignoring the latter one.
The new ErrorDetails code combines multiple details, reusing the
existing Security::ErrorDetail::brief() "a+b+c" syntax.

The new detail accumulation functionality may help detail other errors.

At least some of the logged errno details were lost in commit ba3fe8d.