main is just a sub

[Leopold Toetsch]

Parrot starts execution at the first sub (or that one denoted with 
@MAIN). This subroutine is called with pdd03 calling conventions like 
any other sub. So we have:

   P5   ... argv array
   I0 = 0, I3 = 1   ... one PMC argument passed
A tailcall at the end of main is a valid operation to represent this 
code snippet:

 .main
    ...
    foo()
 .end
as well as a .return() directive (or the omission of one, as missing 
return sequences are inserted by imcc).

The only difference is that a return result of main is *not* promoted to 
the parent process, this can be achieved by the C<exit> opcode.

Please note: PASM code still needs an "end" or the upcoming "returncc" 
opcode.

leo