Add Stdio.File()->accessat() and extend predef::access() to support EUID checks
predef::access() performs access control with the real uid and gid.
Sometimes you want to perform the corresponding checks, but for the effective uid and gid.
predef::access() euid checks may be implemented with
faccessat(2), or emulated via calling of
test(1) with suitable arguments, and should be selected via a third argument containing the character
"e". A suitable detection flag should also be added.
Stdio.File()->accessat() should be implemented with
faccessat(). An availability flag
Stdio.__HAVE_ACCESSAT__ with a value of
1 should also be added when
Stdio.File()->accessat() is available. The symlinks mode of
faccessat(2) should be selectable via the third argument containing the character