add threads test-case, THREADS=2 is ok, bigger numbers crash - so we need to figure out why :)
do not use runtime.SetFinalizer() for now

I can't make it reliable, it looks like the finalizer is called
too early in the TestLibsmbclientThreaded testcase which causes
a crash when other go-routines try to access the Client struct
that got already freed
improve threading test to test both dirs/files
improve multithread test to include multiple Client structs as well
add one global lock :((((