New reproduction cases for the test suite.
Ignore *.vi files, just like *.vo files.
Test suite: keep message in sync with actual file deletions.
Fixing an evar_map bug revealed by commit 603b66f81 on unification flags.

This fixes current failure of RelationAlgebra.
Tentatively more informative report of failure when inferring
pattern-matching predicate.
Fine-tuning unification error (using OccurCheck in evarconv).
Added a CannotSolveConstraint unification error and made experiments
in reporting the chain of causes when unification fails.
Commit not ready. Sorry.

Revert "Fixing an evar_map bug revealed by commit 603b66f81 on unification flags."

This reverts commit d083200ae5b391ceffaa0329a8e3a334036c7968.
First series of results on lists.
List.v: sequel to Sebastien's commit (some cosmetics + a few shorter proofs)
handling Functional Scheme for required but not imported modules
In discrimination nets, do not index lambdas if they're part of a beta
redex.
Two fixes in unification (bugs #3782 and #3709)
- In evarconv, check_conv_record properly computes the parameters of
primitive record projections for later unification, adding env and
sigma as arguments.
- In unification, backtrack on pattern-unification and not only
application unification if eta for a record failed.
Searchxxx now search also the hypothesis and support goal selector.

Documentation also updated.
Extend the syntax of simpl with a delta flag.

You can write 'simpl -[plus minus] div2'. Simpl does not use it for now.
An option SimplIsCbn
Fix #3800 : cmxs need execution priviledges under windows
Fix #3163 and #3843 part 1 : Cygwin DLLs have extension ".so", not ".dll"
#4843 part 2 : The .cmxs files for plug-ins must have execute permission
Searchxxx now interpret patterns in goal environment if any.

This makes such things work:

x:nat
h: x = 3
================
True

Search x.