This fixes a corner case I discovered in the symlink branch, where we
unexpectedly succeed in "replacing" an entire non-empty directory tree
with a file or symlink. This happens when archiving is in use, as we
then just move the entire tree away into the archive. This is wrong as
we should just archive files and fail on non-empty dirs in all cases.
New handling first checks what the (old) thing is, and if it's a
directory or symlink just does the delete, otherwise does conflict
handling or archiving as appropriate.
The number of copiers and pullers is set to default at config loading
time, but the new folder configuration doesn't pass through config
loading so we start up with 0 copiers and 0 pullers and hence get stuck.
I moved the default handling to the puller itself instead. I think this
way is also cleaner as we get to keep the 0 in the config and the puller
gets to decide the defaults on it's own.
