Introduce a new utils_partial_realpath function

This new utils function allows to resolve partially the paths
using realpath. As realpath(3) is not available to use for
unexistent paths, this function that allows to resolve partially
existent paths can be used in cases we don't know if the path
fully exists. It first resolves the existent part using
realpath(3) and then concatenate the unexistent part to it.

Signed-off-by: Raphaël Beamonte <raphael.beamonte@gmail.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>