rename iface/ as spot-if/

So that instead of having to do
  #incluce <spot/iface/ltsmin/ltsmin.hh>
for using installed the installed header, and
  #incluce <iface/ltsmin/ltsmin.hh>
for using the non-installed version, we now do
  #incluce <spot-if/ltsmin/ltsmin.hh>
in both cases.

* iface/: Rename as...
* spot-if/: ... this.
* doc/Doxyfile.in, README, configure.ac, Makefile.am,
spot/sanity/80columns.test, spot/sanity/style.test: Adjust.
* NEWS: Mention the change.
* spot-if/ltsmin/Makefile.am: Install headers in $includedir/spot-if.
* debian/libspot-dev.install: Distribute that directory as well.

configure.ac

@@ -198,9 +198,6 @@ AC_CONFIG_FILES([
   doc/org/.dir-locals.el
   doc/org/init.el
   elisp/Makefile
-  iface/ltsmin/defs
-  iface/ltsmin/Makefile
-  iface/Makefile
   lib/Makefile
   spot/bin/Makefile
   spot/bin/man/Makefile
@@ -220,6 +217,9 @@ AC_CONFIG_FILES([
   spot/twaalgos/gtec/Makefile
   spot/twaalgos/Makefile
   spot/twa/Makefile
+  spot-if/ltsmin/defs
+  spot-if/ltsmin/Makefile
+  spot-if/Makefile
   wrap/Makefile
   wrap/python/ajax/Makefile
   wrap/python/Makefile