The joy of $0. Undoing the #16399 makes Andreas'

The joy of $0.  Undoing the #16399 makes Andreas'
tests (see [perl #22811]) pass (yes, padding with space	instead
of nul makes no sense, but that seems to work, maybe Linux does
some deep magic in ps(1)?); moving the PL_origalen computation
earlier makes also the threaded-first case fully pass.

But in general modifying the argv[] is very non-portable.
(e.g. in Tru64 it seems to be limited to the size of the
original argv[0] since the argv[] are not contiguous?)

Everybody should just have setproctitle().

p4raw-id: //depot/perl@19884