microtime.9: rewrite description, miscellaneous cleanup

microtime.9: rewrite description, miscellaneous cleanup

- Remove the bintime interfaces.  They should not be used outside of
  the timecounting layer.  Unsure whether they warrant a manpage of
  their own.
- In the SYNOPSIS, change the variable names for timespec interfaces
  from "tv" to "ts".
- Document the new-ish "nsec" interfaces.
- Rewrite the DESCRIPTION.  Describe every clock completely in its own
  paragraph.  Enumerate all the interfaces in tables.  Explicitly state
  the output format for each interface in said tables.  Add new vocab
  ("hardware", "timestamp") to clarify the differences between the
  "get" and non-"get" interfaces.
- Add the CONTEXT, RETURN VALUES, and ERRORS sections.
- Cross-reference clock_settime(2), timeradd(3), and tc_init(9).

Lots of input from schwarze@.

ok jmc@ schwarze@