Move prototype parsing related warnings from the 'syntax' top level warnings category...

Move prototype parsing related warnings from the 'syntax' top level warnings category to a new 'illegalproto' subcategory.

Two warnings can be emitted when parsing a prototype -

  Illegal character in prototype for %s : %s
  Prototype after '%c' for %s : %s

The first one is emitted when any invalid character is found, the latter
when further prototype-type stuff is found after a slurpy entry (i.e. valid
character but in such a place as to be a no-op, and therefore likely a bug).

These warnings are distinct from those emitted when a sub is overwritten by
one with a different prototype, and when calls are made to subroutines with
prototypes - those are in the pre-existing sub-category 'prototype'.

Since modules such as signatures.pm and Web::Simple only need to disable
the warnings during parsing, I chose to add a new category containing only
these. Moving these warnings into the 'prototype' sub-category would have
forced authors to disable more warnings than they intended, and the entire
raison d'etre of this patch is to allow the specific warnings involved to
be disabled.

In order to maintain compatibility with existing code, the new location
needed to be a sub-category of 'syntax' - this means that

  no warnings 'syntax';

will continue to work as expected - even in cases like Web::Simple where all
subcategories extant prior to this patch are re-enabled (this is another
reason why a move into the 'protoype' category would not achieve the desired
goal).

The category name 'illegalproto' was chosen because the most common warning
to encounter is the "Illegal character" one, and therefore 'illegalproto'
while minorly inaccurate by ignoring the (relatively recent and unknown)
second warning is an easy name to spot on an initial skim of perllexwarn
and will behave as expected by also disabling the case of an unusual prototype
that happens to look like a normal one.

This patch updates pod/perllexwarn.pod, perldiag.pod and perl5113delta.pod
to document the new category, toke.c and warnings.pl to create and implement
the new category, and a new test t/op/protowarn.t that verifies the new
behaviour in a number of cases. It also includes the files generated by
regen.pl that are found in the repo - notably warnings.h and lib/warnings.pm.