bug#12632: file permissions checking mishandled when setuid

[Eli Zaretskii]

> Date: Sun, 14 Oct 2012 12:42:40 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: 12632@debbugs.gnu.org
> 
>       $ ./a.out eggert eggert/foo games games/foo uucp uucp/foo
>       access(F_OK) euidaccess(F_OK) stat() filename
>              0            0           0    eggert
>              0           -1          -1    eggert/foo
>              0            0           0    games
>             -1            0           0    games/foo
>              0            0           0    uucp
>             -1           -1          -1    uucp/foo
> 
> euidaccess always agrees with ls and with stat, whereas
> access does not.

If euidaccess always agrees with stat, why does it make sense to
switch check_existing to using euidaccess, which is much less portable
and non-standard function, requiring yet another bunch of imports from
gnulib?