Non-nil causes `eval' to abort, unless `inhibit-quit' is non-nil.
If the value is t, that means do an ordinary quit.
If the value equals `throw-on-input', that means quit by throwing
to the tag specified in `throw-on-input'; it's for handling `while-no-input'.
Typing C-g sets `quit-flag' to t, regardless of `inhibit-quit',
but `inhibit-quit' non-nil prevents anything from taking notice of that.