If the last batch had less than plugins.Misc.mores messages;
getting the last messages of the batch would raise an IndexError,
causing the whole batch to be dropped.
It crashes with KeyError because databases.plugins.channelspecific sets
channel='#', which is a non-existent channel.
And it doesn't make sense to check for presence in channel if the DB is
shared between all channels anyway.
I don't see any use for this; and it means a 'channel' converter
called after 'channeldb' returns '#' instead of the actual channel
if it's after 'channeldb' in the arg list and
databases.plugins.channelspecific is False.
To fixes a mismatch between callbacks, which use flags=re.I by default,
and SED_REGEX.search which isn't; so 'S/foo/bar/' errored because it
matches case-insensitively so _unpack_sed is entered, but then _unpack_sed
fails to match it case-sensitively.
The connect() method already adds it, so it was in the list twice
(added both by __init__() and connect()).
This caused _select() to call _read() twice on the same instance,
except there is usually nothing to read on the second call,
so it blocks for up to conf.supybot.drivers.poll().
Fixes c1d3bad64f, which crashed with:
```
ERROR 2020-01-18T01:34:07 Uncaught exception in NickAuth._callCommand:
Traceback (most recent call last):
File "/usr/local/lib/python3.7/dist-packages/supybot/callbacks.py", line 1337, in _callCommand
irc.errorNoCapability(cap)
File "/usr/local/lib/python3.7/dist-packages/supybot/callbacks.py", line 544, in errorNoCapability
raise Error()
supybot.callbacks.Error
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/usr/local/lib/python3.7/dist-packages/supybot/log.py", line 368, in m
return f(self, *args, **kwargs)
File "/usr/local/lib/python3.7/dist-packages/supybot/callbacks.py", line 1359, in _callCommand
irc.error(str(e))
File "/usr/local/lib/python3.7/dist-packages/supybot/callbacks.py", line 1065, in error
raise ArgumentError
supybot.callbacks.ArgumentError
```
To prevent infinite recursion on startup, because defaultHttpHeaders
gets the value; which is unset, so 'registry._lastModified > self._lastModified'
is True, so .set() is called, so .setValue() is called again.
