sys/time.h: add bintime2us() helper

Add a microsecond conversion helper to complement the existing
bintime2ns(). The body mirrors bintime2ns().
This will be used by an upcoming eventlog(9) framework as well as
the TCP code in upcoming changes.

Approved by: gallatin, tuexen
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D56972