Fix scope problem when using the return value of get_root_dir()

Fix scope problem when using the return value of get_root_dir()

Let get_root_dir return a reference instead of a temporary
std::string object.  In directory_is_absolute() and
directory_is_rootdir(), use std::string methods rather than
falling back to plain C techniques.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>