Fix up ARTV import so that imports actually work.

Fix up ARTV import so that imports actually work.
Fixed various client_getln loops so that they don't go endless if the
client disappears.
ARTV import now makes use of the internal base64 decoder so it should be
quicker.
ARTV import now sends lots of dots to the client. This is to kick the
watchdog timer in sendcommand so that it doesn't expire while we process
a big message.
Changes in threads.* are in preperation for cancelling threads and using
signals to interrupt threads. They are #ifdef excluded.