efl vpath: Check and terminate execution if set*uid() calls fail

efl vpath: Check and terminate execution if set*uid() calls fail

Summary:
Quells warnings:
    lib/efl/interfaces/efl_vpath_core.c:117:9: warning: ignoring return
    value of ‘setuid’, declared with attribute warn_unused_result
    [-Wunused-result]
         setuid(geteuid());
         ^
    lib/efl/interfaces/efl_vpath_core.c:169:9: warning: ignoring return
    value of ‘setreuid’, declared with attribute warn_unused_result
    [-Wunused-result]
         setreuid(uid, geteuid());
         ^

Reviewers: raster, jpeg

Reviewed By: raster

Subscribers: cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D4768