Avoid multiple cscope process for one true path

Use absolute path for rscope buffer to avoid having two processes for ~/
and /home/$USERNAME paths

Signed-off-by: Sylvain Chouleur <sylvain.chouleur@gmail.com>
Fix non killed process in rscope-init

When calling multiple times rscope-init for the same directory, some
processes were not (and so never) killed.

'get-process function take a process name as argument, not a buffer
name. Use 'get-buffer-process instead.

Signed-off-by: Sylvain Chouleur <sylvain.chouleur@gmail.com>