flymake: fall back to margins on text terminals

flymake: fall back to margins on text terminals

Previously, flymake-indicator-type defaulted to either fringes
or margins.  But fringes should be used on graphical frames, and
margins on TTY frames.  So default to fringes instead, and
simply fall back to margins automatically on text frames.

* lisp/progmodes/flymake.el (flymake-indicator-type): Set to
fringes.  (bug#77313)
(flymake-mode): Fallback to margins if there's no fringes.
* doc/misc/flymake.texi (Customizable variables): Mention
fallback behavior.
* etc/NEWS: Announce fallback behavior.

(cherry picked from commit 861e7f8b60e4bf076bf5991d25a22b3a012746bd)