rename SB-SIMPLE-STREAMS utility function

rename SB-SIMPLE-STREAMS utility function

At least, I think it was simply intended to be a utility function,
but the FILE-NAMESTRING name is unfortunate, as that's a CL function,
and (for other reasons) SB-SIMPLE-STREAMS claims to implement CL in
the package lock sense, so this was not noticed at the time.

This fixes lp#884603; I am unconvinced that the namestring logic is
actually correct (keeping the distinction between NAMESTRING and
NATIVE-NAMESTRING clear) but I'll wait for an actual user to complain
about that before thinking too hard.