[perl #118565] Illegal character after '_' in prototype - warning is improper

Attaching a patch for this.

It turns a maximum of 2 warnings into a maximum of 3 warnings on a bad
prototype, updates perldiag to add a little more detail on "after '_'"
is its own category, and of course adds tests.

And of course, in the scope of a 'no warnings "illegalproto"', its still 0.

• [perl #118565] Illegal character after '_' in prototype - warning is improper by Father Chrysostomos via RT
• [perl #118565] Illegal character after '_' in prototype - warning is improper by Peter Martini via RT
• [perl #118565] Illegal character after '_' in prototype - warning is improper by Peter Martini via RT