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