Don't call varargs open() through non-varargs type

open() is probably defined varargs.  Casting a varargs function to an
equivalent non-varargs type and then calling it is undefined, but
gfileutils.c was doing exactly that.

Add some non-varargs wrappers to avoid the problem.

Problem reported by John Spencer.

https://bugzilla.gnome.org/show_bug.cgi?id=687600