smbd: Use mutex instead of fcntl lock for echohandler coordination

smbd: Use mutex instead of fcntl lock for echohandler coordination

This is based on a patch from Volker. When the system supports roboust
mutexes, they will be used for the coordiations between worker and
echohandler process. This avoids another aspect of the fcntl scalibility
issue when handling many client connections. When mutexes are not
available, the code falls back to the fcntl lock.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Jul  9 00:56:50 CEST 2014 on sn-devel-104