Detail client closures of CONNECT tunnels during TLS handshake (#691)

Detail client closures of CONNECT tunnels during TLS handshake (#691)

... and improve detailing of other errors.

Many admins cannot triage TLS client failures, and even Squid developers
often cannot diagnose TLS problems without requiring detailed debugging
logs of failing transactions. The problem is especially bad for busy
proxies where debugging individual transactions is often impractical.

We enhance existing error detailing code so that more information is
logged via the existing %err_code/%err_detail logformat codes.
Propagating low-level error details required significant enhancements
and refactoring. We also built initial scaffolding for better error
detailing by GnuTLS-driven code and documented several key
error-handling APIs, exposing a few out-of-scope problems.

Also checkLogging() once, after consuming unparsed input attributed to a
transaction: Due to fake CONNECT requests, from-client read errors, and
possibly other complications, we may have a transaction that did not
consume every input byte available to it. That transaction is still
responsible for reporting those unparsed bytes (e.g., by logging the
number of bytes read on a connection and the number of parsed bytes).

Also fixed passing wrong (errno vs. size) or stale (requested vs. read)
I/O size to connFinishedWithConn(); now shouldCloseOnEof(). The bad
value was "correct" (i.e. zero) in many cases, obscuring the bug.

This is a Measurement Factory project