Re: new feature: s?printf parameter reordering

Re: new feature: s?printf parameter reordering
Message-Id: <200101111709.RAA23756@crypt.compulink.co.uk>

- support reordering for all parameters: %, *v, *, .*
- lay down that the reordering specification must immediately
  follow that parameter: %3$, *v3$, *3$, .*3$
- fix vectorisation of a zero-length string
- factor out the code choosing the argument to format

Possibly unwanted side-effects:
- the special format specifiers ' +-0' must now precede any
  vectorisation specifier. Tests in op/sprintf and op/ver
  have been changed to reflect this.
- sprintf.t test #214 changed its expectations because in many
  cases, the next parameter has already been consumed when an
  invalid type letter is detected.

Probably wanted side-effects:
- attempts to format a non-existent parameter will warn as if C<undef>
- attempt to write to non-existent parameter with '%n' will complain
  of "attempt to modify read-only value" instead of being silent

p4raw-id: //depot/perl@8481