bug#371: describe-* and M-x should see past quote mark

[jidanni]

If the cursor is upon the ' in
'emacs-version'
describe-variable should still prompt for emacs-version.

E.g., on many Info pages there are lists of variables
`w3m-command-arguments'
'gnus-version'
one should be able to use C-n to slide down the edge without needing
to advance the cursor one step in order to use C-h v.
(OK, one could slide down column 1, but C-v puts us back in column 0.
Don't (not) worry, I will file a bug.)

Same detection algorithm should apply to C-h f and M-x.