[ast-developers] Use posix exit code if last command exits due to a signal

[Siteshwar Vashisht]

If the last command in a script was killed by a signal, ksh kills itself with the same signal. Attached patch fixes it by using exit() (with status 128 + signal number) instead of kill(). It also fixes the bug where ksh kills itself with a signal if argument to exit command is greater than 256.

GitHub Pull Request: https://github.com/att/ast/pull/57