rationalise subroutine parsing rules

Now that the parser rules have been split into separate rules for subs
under 'use feature "signatures"' and not, refine the rules to reflect the
different regimes. In particular:

1) no longer include 'proto' in the signature variants: as it happens the
toker would never return a proto THING under signatures anyway, but
removing it from the grammar makes it clearer what's expected and not
expected.

2) Remove 'subsignature' from non-sig rules: what used to happen before
was that outside of 'use feature "signatures"', it might still try to
parse a signature, e.g.

    $ perl5279 -we 'sub f :lvalue ($$@) { $x = 1 }'
    Illegal character following sigil in a subroutine signature at -e line
    1, near "($"
    syntax error at -e line 1, near "$$@"

Now it's just a plain syntax error.

1 files changed, 1 insertions, 1 deletions

diff --git a/perly.h b/perly.h
index 50c0eb3fb5..66771226e1 100644
--- a/perly.h
+++ b/perly.h
@@ -183,6 +183,6 @@ int yyparse (void);
 
 
 /* Generated from:
- * 7d1ae56dfb5e7feea413f76193d80121912cb6aadf882158a70abc41e0153b1a perly.y
+ * 1464f62136e1c8980247e644a1b280abdab234c9615dcbc70e089ab38123122b perly.y
  * b6fae5748f9bef6db4740aa5e122b84ac5181852d42474d0ecad621fa4253306 regen_perly.pl
  * ex: set ro: */