updater_runner.py: reload win32_updater_channel each time

Reload win32_updater_channel each time, so one does not need to
restart Neubot after this setting has been changed in order to
make it effective.
updater_runner.py: deal with recent bigmerge fallbacks

During recent bigmerge I did some mistakes and the result was
that some bits of change were lost.
updater_runner.py: remove unused import
add updater_verify.py: verify autoupdater tarball via openssl
updater_verify.py: portably join paths
log.py: remove extra blank line
log.py: ROOT -> ROOT_LOGGER for consistency
log.py: stop passing useless DEBUG flag to StreamLogger

Filtering is performed by the root logger before even reaching
the handler.  So, do no specify any level for StreamLogger, and
make the root logger be always verbose.

Of course, this breaks the -v flag, but repairs the streaming
logger feature.  I'll repair the -v flag with next diffs.
log.py config.py: preparatory work to repair verbose feature

Now log_verbose is a CONFIG setting.  When this setting is true,
DEBUG messages are logged.  Otherwise they are not logged.

At the same time, the "root" logger is configured to be always
verbose.  Hence, the streaming handler receives all log messages,
and the streaming feature works as expected.
config.py: implement __setitem__()

Long-awaited feature to make it easier to deal with the main
configuration object.

This commit is preparatory work to repair the -v feature, which
was broken after latest round of log.py cleanups.
Repair -v: part 1

This diff repairs the -v command line switch for some Neubot
subcommands.  This is just part of the work.  Subsequent diffs
will finish the work.
MD5 changes in PKGBUILD
Fixed Archlinux rc.d file
Repair -v: part 2

This diff repairs the -v command line switch for the Neubot
subcommands that have not been addressed by previous diff.
Repair -v: don't always invoke runner client in verbose mode

Previous diffs passed True to runner client, resulting in an always
verbose runner client.  This is way too annoying.  Much better to
pass it the current value of CONFIG['verbose'] instead.

Now I think I have repaired -v.  And, more importantly, now, also,
the streaming logger should work as expected.
log: stream -> streaming for clarity

The streaming feature deals with passing a client the logs
generated by the neubot daemon while running a test on behalf
of such client.  It has nothing to do with "stream" logging,
where stream here is intended as a stdlib file.

To avoid confusion between stream and streaming, make sure
that names related to streaming feature use the "streaming"
prefix, not the "stream" prefix.
log.py: document extensively the streaming feature
streaming log: filter ACCESS logs in runner client

This is typical UNIX-zen.  The core code is simple and generic
and filtering is implemented at the edges.  Of course, this
is something that may cause performance problems in some cases.
E.g. when you are passing stuff that typically is discarded,
and passing along that stuff causes significant slow downs.

In this respect, sometimes I happen to worry whether this
streaming feature may slow down tests.  But, in any case,
access logs are not something that should increase too much
the load.  Since the bulk of log messages are DEBUGs, and
they were already passed to the client.
log.py: remove ultra-paranoid code

Of course we're not going to keep the exception alive, unless
we save it somewhere.  I think I wrote that very paranoid piece
of code late in the night.  When I'm more likely to mistake,
and so I tend to deploy excessive robustness (sometimes).
log.py: don't clear args after lazy string interpolation

When this code was a single function it made sense to do that,
to avoid to do string interpolation more than once.  Now it
has become pointless, after Marco's refactoring work, so it's
safe to dispose of this code.