flow.cfg et al: Accept "u" as the micro(seconds) input in cfg::Option_set (and for general istreams?)

[ygoldfeld]

flow.cfg et al: Accept "u" as the micro(seconds) input in cfg::Option_set (and for general istreams?)

Filed by @ygoldfeld pre-open-source:

At some point we made it possible to affect an ostream or Logger (and made it default for flow::async-created threads) in such a way as to output "us" for microseconds instead of Unicode mu-s.

Symmetrically, we should enable accepting the same as input, in flow::cfg and for arbitrary istreams.

This is low-priority; no one relies in practice on accepting mu-s in parsing input, as Unicode is a pain in all our contexts. However, for completeness, this should be done.