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.

spot-if/ltsmin/finite.pm

@@ -0,0 +1,8 @@
+active proctype P() {
+int a = 0;
+int b = 0;
+x: if
+  :: d_step {a < 3 && b < 3; a = a + 1; } goto x;
+  :: d_step {a < 3 && b < 3; b = b + 1; } goto x;
+fi;
+}