Fixing file destruct.v.
Fixing 1177da327 (reorganization of the test for generic selection of
occurrences: some uniformisation was not appropriate for "change").
Cosmetic changes.
Plural and singular forms in error messages.
Supporting "at occs" as a short-hand for "in |- * at occs" in "destruct".
Fixing subterm matched for destruct when it is matched from prefix.
Some reorganization of the code for destruct/induction:

- It removes some redundancies.
- It fixes failures when destructing a term mentioning goal hypotheses
  x1, ..., xn or a term which is a section variable x when this term
  is in a type with indices, and the indices also occur in the type of
  one of x1, ..., xn, or of x.
- It fixes non-respect of eliminator type when completing pattern possibly
  given by a prefix.
- It fixes b2e1d4ea930c which was dealing with the case when the term
  was a section variable (it was unfortunately also breaking some
  compatibility about which variables variable were generalized in
  induction and which variables were automatically cleared because
  unselected).
Improving elimination with indices, getting rid of intrusive residual
local definitions...
Saving some nf_evars in the code of destruct/induction.