Improve on previous quote autopairing change

Improve on previous quote autopairing change

Backported from trunk r116940

* lisp/elec-pair.el:
(electric-pair--syntax-ppss): When inside comments parse from
comment beginning.
(electric-pair--balance-info): Fix typo in comment.
(electric-pair--in-unterminated-string-p): Delete.
(electric-pair--unbalanced-strings-p): New function.
(electric-pair-string-bound-function): New var.
(electric-pair-inhibit-if-helps-balance): Decide quote pairing
according to `electric-pair--in-unterminated-string-p'

* test/automated/electric-tests.el (define-electric-pair-test): Don't
overtest..
(inhibit-in-mismatched-string-inside-ruby-comments): New test.
(inhibit-in-mismatched-string-inside-c-comments): New test.