top(1): use a different command to toggle tid vs pid

top(1): use a different command to toggle tid vs pid

- By popular demand, implement a different switch ("T") for toggling
between thread id and process id.
- Add an assert that the size of command chars is as expected.
- Also clean up some messiness I found when implementing this.
- Further document the new flag.

Requested by:	flo, ronald-lists@klop.ws, bapt
PR:		139389 (for the record)
X-MFC-With:	r334474