Set O_NOATIME flag on Linux

Citing Kerrisk, The Linux Programming Interface:

    The O_NOATIME flag is intended for use by indexing and backup
    programs. Its use can significantly reduce the amount of disk
    activity, because repeated disk seeks back and forth across the
    disk are not required to read the contents of a file and to update
    the last access time in the file’s i-node[.]

restic used to do this, but the functionality was removed along with the
fadvise call in #670.

internal/fs/setflags_other.go

@@ -0,0 +1,12 @@
+//go:build !linux
+// +build !linux
+
+package fs
+
+import "os"
+
+// OS-specific replacements of setFlags can set file status flags
+// that improve I/O performance.
+func setFlags(*os.File) error {
+	return nil
+}