when asking confirmation, retry on unknown input

Before:

  $ opam install why3; echo "finished"
  The following actions will be performed:
   - install zarith.1.2 [required by why3]
   - install coq.8.4pl2 [required by why3]
   - install lablgtk.2.16.0 [required by why3]
   - install ocamlgraph.1.8.3 [required by why3]
   - install alt-ergo.0.95.2 [required by why3]
   - install why3.0.81
  6 to install | 3 to reinstall | 0 to upgrade | 0 to downgrade | 0 to remove
  Do you want to continue ? [Y/n] toto
  finished

After:

  $ opam install why3; echo "finished"
  The following actions will be performed:
   - install zarith.1.2 [required by why3]
   - install coq.8.4pl2 [required by why3]
   - install lablgtk.2.16.0 [required by why3]
   - install ocamlgraph.1.8.3 [required by why3]
   - install alt-ergo.0.95.2 [required by why3]
   - install why3.0.81
  6 to install | 3 to reinstall | 0 to upgrade | 0 to downgrade | 0 to remove
  Do you want to continue ? [Y/n] toto
  Do you want to continue ? [Y/n]

The list of considered-unambiguous answers was also extended to
include "Yes" and "No", for example.
Merge pull request #1137 from gasche/master

when asking confirmation, retry on unknown input