Currently the code to convert `int err` or errno into string is usde
only in _pathError, but in the next patches we'll need it to also handle
error from pthread_atfork. -> Factor-out to separate function.

ZEO4 can have ._addr as either ("host", port) or "path".

People don't want to raise up soft defaults
nexedi/slapos.package!130 (comment 118373)
nexedi/slapos.package!130 (comment 118405)

Previously if an assert or something failed in spawned thread, the main
thread was usually spinning indefinitely = tests hang. -> Switch all
threading places to use sync.WorkGroup and this way if a thread fails,
all other threads are canceled and the exception is reported back to
wg.wait in main thread.

This should cure the situation when on testnodes /tmp/wcfs already exists
created without sticky bit.

So that multiple users can create things inside.