Returns the file extension appended to the
names of modified files under in-place edit mode. This value can be set
using ARGF.inplace_mode= or passing the -i switch to the
Ruby binary.
static VALUE
argf_inplace_mode_get(VALUE argf)
{
if (!ARGF.inplace) return Qnil;
if (NIL_P(ARGF.inplace)) return rb_str_new(0, 0);
return rb_str_dup(ARGF.inplace);
}