Do it all at once in preparation for Go 1.18 support.
To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.
readdir is disabled on linux for 386 and arm until syscall.seek is implemented there.
windows is hard, so leaving that for later.
File src/os/dir_other_go115.go can be deleted when we drop support for go 1.15.
Also adds TestReadNonDir, which was helpful while debugging.
