program exit code

[Leopold Toetsch]

Parrot programs have commandline info in P0 but there is no means to 
communicate an exit-status to the shell.
We could do:

1) REG_INT(5) ...has exit code
2) end Ix ... end opcode has exit code
3) exit_code Ix .. set exit code
1) breaks existing programs but fits calling conventions
2) could break tools, which assume C<end> is opcode #0
so probably 3) or ...
Comments welcome
leo