rscope: add cpp files to automatic cscope.out generation

Add C++ source files to automatic cscope database generation.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
rscope: add key to manually regenerate cscope.out database

Add a new key to regenerate the cscope database. As rscope is not
automatically regenerating the database, a new key enable to recompute
the cscope.files and cscope.out.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
rscope: fix rscope buffer killing on cscope error

Fix a small bug in rscope buffer killing.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
rscope: cleanups

Do basic cleanups :
 - common query calls
 - byte-compile warnings

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>