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/sanity/80columns.test

@@ -42,7 +42,7 @@ fi
 x="$x$x$x$x$x$x$x$x$x" # 9x
 x="$x$x$x$x$x$x$x$x$x" # 81x
 
-for dir in "${INCDIR-..}" "${INCDIR-..}"/../iface; do
+for dir in "${INCDIR-..}" "${INCDIR-..}"/../spot-if; do
 
   find "$dir" \(    -name "${1-*}.hh" \
                  -o -name "${1-*}.hxx" \

spot/sanity/style.test

@@ -52,7 +52,7 @@ export LC_ALL
 
 tmp=incltest.tmp
 
-for dir in "${INCDIR-..}" "${INCDIR-..}"/../iface; do
+for dir in "${INCDIR-..}" "${INCDIR-..}"/../spot-if; do
 
   find "$dir" \(    -name "${1-*}.hh" \
                  -o -name "${1-*}.hxx" \
@@ -326,7 +326,7 @@ done
 
 
 # Rules for Makefiles.
-for dir in "${INCDIR-..}" "${INCDIR-..}"/../iface; do
+for dir in "${INCDIR-..}" "${INCDIR-..}"/../spot-if; do
 
   find "$dir" -name "Makefile.am" -a -type f -a -print |
   while read file; do