lib/path: add ul_path_stat(), fix absolute paths

lib/path: add ul_path_stat(), fix absolute paths

* add ul_path_stat()

* make sure all paths for ul_path_..() functions are always
interpreted relatively to the context directory and prefix. This is
difference between ul_path_ API and standard libc "at" functions. We
do not use any exception for absolute paths. The reason is that we
need to read from prefixed paths although application assume absolute
path (/dev/sda1 means /prefix/dev/sda1 if a /prefix is defined).

Signed-off-by: Karel Zak <kzak@redhat.com>