Without special care a forked child may interfere in parent-wcfs
exchange via Python GC -> PyFileH.__del__ -> FileH.close -> message to
WCFS sent from the child. This actually happens for real when running
test.py/neo-wcfs because NEO test cluster spawns master and storage
nodes with just fork without exec.

-> detach from wcfs in child right after fork and deactivate all
mappings in order not to provide stale data. See top-level comments
added to wcfs/client/wcfs.cpp for details.

Currently in wcfs_test.py there is only waiting for a proc
(subprocess.Popen instance) to become ready. However in the next patch
we'll need to wait via polling for another condition.

-> Generalize the pollwait code into waitfor* variants, and make
procwait* use waitfor* internally.

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.