In preparation for switching linuxulator to the use the native 1:1

Description

In preparation for switching linuxulator to the use the native 1:1
threads introduce linux_exit() stub instead of sys_exit() call
(which terminates process).
In the new linuxulator exit() system call terminates the calling
thread (not a whole process).

Differential Revision: https://reviews.freebsd.org/D1027
Reviewed by: trasz