(eldoc-minor-mode-string): Add autoload cookie; otherwise autoloaded

(eldoc-minor-mode-string): Add autoload cookie; otherwise autoloaded
add-minor-mode call complains in Emacs 21.
Use add-minor-mode to set minor-mode-alist, if available.

(eldoc-echo-area-use-multiline-p): New user option.
(eldoc-echo-area-multiline-supported-p): New variable.
(eldoc-docstring-format-sym-doc): Use them.
(eldoc-mode): If not using idle timers, append to local post and
pre command hooks.  Suggested by David Byers <davby@ida.liu.se>.

(eldoc-display-message-no-interference-p): Don't interfere with edebug.
Add autoload cookie for eldoc-mode minor-mode-alist initialization.

(eldoc-function-arglist): New function.
(eldoc-function-argstring): Use it.