Centralize the definition of frame flags

[wtarreau]

The state machine diagram currently mentions EH/ES but each frame has its own definition which happens to always have the same value. Let's instead enumerate known flags with their values at one place, and mention for each frame which ones it supports or are forbidden. This will significantly simplify implementations and will cut the text. It might even lead to generalizing flag processing instead of repeating the text for each frame type.