Split the handling for "Illegal character" and "Illegal character after '_'"

After applying this patch, both can now be triggered at once, whereas
previously any use of '_' would trigger the "after '_'" variant.  Since
the two warnings warn for different reasons, there's no reason to
conflate the two.  Also updated perldiag with a clearer explanation
of the tighter restrictions after an underscore.

(In the tests, the change of uniproto12 to uniproto13 is merely correcting
an error from a previous patch, reusing the name would conflate
two kinds of tests in one statement).