The user should not usually be interested in internal coroutines, at least
unless they refuse to stop.

Also show error code in some pending cases.

To avoid boilerplate in messages logged via `Yield`, also for abort message on
exception at top level.

This also fixes commit b583c0e1 in case the client has debugging off.

A common prefix appears in all plain messages, and no prefix (just the tag)
appears in messages logged via `Yield`.

If missing, stay with `INFO` for backwards compatibility.

Also, detect early when the given level is disabled, and avoid doing string
conversion and line splitting.

And remove one unused such function.

Same as with client, this is handled elsewhere (client front-end).

And setting it from client front-end.  The announcer still needs to get its
previously inherited log level setting removed.

This is handled elsewhere (client front-end), the client should not debug if
debugging is off, regardless of whether logging to a file or not.

There has not been information worth refreshing on the front-end for a while,
and it makes using its controls quite tricky.

With download link.

Recover the log level once the log file is disabled (unless explicitly set by
the user while logging to the file).

By copying the key which may become trashed on cancellation (when debugging).

So that it does produce output if the log level enables debugging during
runtime.

This adds a link to the client front-end to retrieve a list of groups being
announced by this client.  Directly available as `/groups.txt` from the
front-end.

To avid getting in the middle of other text.

As plain text, one per line, undefined ordering.

This release fixes some build errors in Android and a wrong assertion which
may interfere with debug builds.

Fix some issues with the Android build.

It triggers an error in the Android build.

To avoid compiler warning for Android.

This release brings a new implementation of the multi-peer download protocol
with some fixes to response injection.  The new versions of signing and
storage protocols are interim until new versions cleaned up for consistency
are released.

Enhancements:

  - New protocol v6 supporting multiple, concurrent, client-to-client
    download.  As computation of block signatures also changes, this implies a
    new storage format v3.

Bugfixes:

  - Fix injection of responses with an empty body.  The block signature
    created by the injector was incorrect and caused other clients to get
    stuck when retrieving such resources from other clients.  This was
    specific to v5 injector implementations.

    Please note that this affected redirections in the landing pages of some
    websites (e.g. `example.com -> https://www.example.com/`), which rendered
    the sites unfit for retrieval from other clients.

This includes a new working implementation of multi-peer downloads using a
two-phase protocol for the concurrent retrieval of content seeded by several
other clients.

Ouinet protocol v6 and signed HTTP storage v3 are introduced.  They both are
interim verions until some cleanups afecting protocol consistency are applied,
specs updated, and v7 is released.

They do not work as protocol v6 requires a previous retrieval of hashes and
signatures (without data).  This needs to be fixed by v7.  Look for `TODOv6`
and `TODOv7`.

Since the way blocks are signed is different and client-to-client retrieval
requires a first exchange of block signatures and hashes.

Since the meaning of chained hashes in `sigs` files has changed.

Updates will come later, look for `TODOv6` or `TODOv7`.