_IO_file_underflow_mmap read call for atime

[Roland McGrath]

> Fine, cannot we mmap at the time of the first read then (and if fall back to
> non-mmap stdio if that is not possible)?

That seems more desireable to me anyway.  As it stands, a program that just
calls fopen and then fileno and does not io, or just uses fopen+fclose to
test existence (I have seen such things) will map in the contents of the
file when that's never required at all.