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.
