Adjust flymake integration

Adjust flymake integration

When opening a new file (signalling textDocument/didOpen) it makes
sense to call the flymake callback (if it exists) with no diagnostics,
just to get rid of that "Wait", since we don't know if later in this
callback cycle the server will ever report new diagnostics.

* eglot.el (eglot--managed-mode): Don't call flymake-mode or eldoc-mode
(eglot--managed-mode-hook): Add them here.
(eglot--maybe-activate-editing-mode): Call flymake callback.
(eglot--server-textDocument/publishDiagnostics): Set unreported
diagnostics to nil if invoking callback.