src/runas: Fixes and cleanups

The runas helper runs a command as another user and/or with different group
memberships.  Fix the following problems:

 * Use setgid instead of setegid and setuid instead of seteuid.
   Otherwise, the command will run with the original real UID
   and/or GID; those could be made the effective IDs again.

 * When only a GID is specified, remove all supplementary
   GIDs.  Otherwise, the command would remain in the same
   supplementary groups as runas -- which often is the root
   group.

 * Use execvp instead of execv which searches the PATH when
   necessary.  The runas helper is always called either with a
   '/' in the pathname or as "runas ... `which program`", so
   we obviously want PATH lookup, anyway.

 * There is no advantage in fork'ing and waiting for the child
   over directly exec'ing the command; the test cases already
   have to deal with commands which can be killed by signals.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/shared/051

@@ -294,10 +294,10 @@ done
 popd >/dev/null
 chown -R 12345.54321 root
 echo "Change #1..."
-$runas -u 12345 -g 54321 -- `which chacl` -r u::rwx,g::-w-,o::--x root
+$runas -u 12345 -g 54321 -- chacl -r u::rwx,g::-w-,o::--x root
 find root -print | xargs chacl -l
 echo "Change #2..."
-$runas -u 12345 -g 54321 -- `which chacl` -r u::---,g::---,o::--- root
+$runas -u 12345 -g 54321 -- chacl -r u::---,g::---,o::--- root
 find root -print | xargs chacl -l
 
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