use _exit instead of exit if client fails to exec

use _exit instead of exit if client fails to exec

exit() calls atexit() handlers and C++ destructors (e.g. a C++
weston module) which could destroy state that the main process
depends on (e.g. ioctl's, tmpfiles, sockets, etc...).  If an exec
fails, call _exit() instead of exit().

v2: prefer _exit over _Exit

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>