vcl: handle pthread cleanups in vls

vcl: handle pthread cleanups in vls

For historic reasons vcl registers for atexit and pthread exit cleanup
callback functions. Since vls is the one handling thread detection, move
thread cleanup logic there as well.

This also avoids potential issues if main/first pthreads exists before
the subsequent spawned ones as it may lead to premature vcl worker
cleanup.

Type: improvement

Change-Id: Id7a5c186b48f1e4c60ced635d918b5d4b4143fa6
Signed-off-by: Florin Coras <fcoras@cisco.com>