h-entry validator should check for valid RSVP values!

h-entry validator should detect if an h-entry is an RSVP (e.g. has p-rsvp property) and make sure the value provided is valid (yes,no,maybe,interested).

Maybe suggest markup if an invalid value is provided e.g. if someone says
```
I'm attending
```
perhaps suggest instead
```
I'm attending
```
cc @aaronpk (who discovered invalid RSVP values received by webmention.io)

Start at the h-entry validator entry point and trace how p-rsvp properties are detected and their values are interpreted. Check the four allowed values—yes, no, maybe, and interested—and review how the validator reports invalid markup. Done means valid RSVP values pass and invalid values are flagged, with the suggested data markup considered if supported.

Written by the indexing model from the issue text.