eina_vpath: fix homedirectory fetching
the fetching of the homedirectorty did not work. pw_dir was never
prepended. Additionally you would get a silent NULL string back
if the system does not support HAVE_GETPWENT.
Depends on D6737
Reviewed By: zmike
Maniphest Tasks: T7107
Differential Revision: https://phab.enlightenment.org/D6742