Have the VMS exit code follow POSIX conventions

It seems like the convention for VMS exit codes is to combine the VMS
C facility code (0x35a000) with a recoded exit code as follows:

    0     => 1
    1-255 => 8*code + 2

We also add 0x10000000, which is the control bit that has DCL not
report the error on the terminal.  That's just as well, since it would
be quite nonsensical, for example:

    %C-W-NOMSG, Message number 0035A018

We could do all this by using the normal exit() function after having
defined the macro _POSIX_EXIT.  Unfortunately, this feature only
exists in VMS C V7.1 and up.

Reviewed-by: Rich Salz <rsalz@openssl.org>