parasite: allow to wait more than one process on the exit from syscall

All processes must be started by PTRACE_SYSCALL. The function calls wait
in a loop and if a process on the exit from the required syscall, it
is stopped, otherwise it will be reexecuted by PTRACE_SYSCALL.

The function doesn't know, which processes should be trapped, so
you should care, that wait() will not catch someone else.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>