Allow ending spaces to be > 65535

[bengtmartensson]

Currently, when using uint16_t as microseconds_t, ending spaces are limited to that data type. This is not enough for many use cases.

Requirement on a solution is that it does not depend on microseconds_t being uint16_t,

Branch ExtraEndingGap. millisec-edning

[bengtmartensson]

Alternative (better?) is to have microseconds_t scaled, for example by 2.

[bengtmartensson]

In the light of #11 (fixed), this is no longer a "bug", but a limitation.