bug#14710: add-file-local-variable vs. unquoted string

[Juri Linkov]

> Good point.  I fixed it with the patch below, which uses
> read-from-minibuffer (with a non-nil `read' argument) instead of
> read-string, so it re-uses the check that was already used when reading
> an Elisp expression.

FWIW, when I wrote `read-file-local-variable-value' I copied its code:

      (read (read-string (format "Add %s with value (use quotes for strings): "
                                 variable)
                         nil 'set-variable-value-history
                         (format "%S" ...

from `set-variable':

                   (read
                    (read-string prompt nil
                                 'set-variable-value-history
                                 (format "%S" ...

So `set-variable' seems to have the same problem, although
after entering a string without quotes with e.g.
`M-x set-variable RET compile-command RET a b c RET'
it checks `custom-type' of the entered value and reports the
error "Value `a' does not match type string of compile-command".

Perhaps this is not too problematic as long as `set-variable' is used
only for customizable options, and not for ordinary variables.