Do not unmount /proc/sys/fs/binfmt_misc before runnint the check scripts

We did this to work around a bug in an old test script, but it
breaks building with newer kernels. So now detect the presense
of the broken check script and only unmount in that case.

Fixes bsc#1219137

build

@@ -1914,7 +1914,9 @@ TIME_POSTCHECKS=$SECONDS
 if test -n "$RPMS" -a -d "$BUILD_ROOT/usr/lib/build/checks" ; then
     # workaround for broken 13.1 check scripts which umount /proc
     if test -n "$RUNNING_IN_VM" -a "$BUILD_ROOT" = / ; then
-        umount -n $BUILD_ROOT/proc/sys/fs/binfmt_misc 2>/dev/null
+	if grep umount "$BUILD_ROOT/usr/lib/build/checks/99-check-remove-rpms" >/dev/null 2>&1 ; then
+	    umount -n $BUILD_ROOT/proc/sys/fs/binfmt_misc 2>/dev/null
+	fi
     fi
     # find package name
     export PNAME=