Allow identity mapping between os_time_t and microsoconds.

[terrillmoore]

The inexact relationship between os_time_t and Arduino micros() is often maddening and wasteful.

A code review suggests that we could change code pretty simply to allow os_time_t to be microseconds on platforms where that's convenient. It's just a matter of finding all the places where the code scales by US_PER_OSTICK_EXPONENT and related values, and make sure things work when it's set to 0.

This would reduce the dynamic range of os_time_t from 19 hours to 1 hour. We'd need to review whether the LMIC needs to deal with times longer than 1 hour internally. This might affect sketches that use os_time_t for long term scheduling (by submitting an LMIC job).