time-util: accept "µs" as time unit, in addition to "us" (#4836)

Let's accept "µs" as alternative time unit for microseconds. We already accept
"us" and "usec" for them, lets extend on this and accept the proper scientific
unit specification too.

We will never output this as time unit, but it's fine to accept it, after all
we are pretty permissive with time units already.