Change name of Timeout key familly.

Rename TimeoutKill to TimeoutStart.
Rename TimeoutFinish to TimeoutStop.
Remove TimeoutUp and TimeoutDown key.
To avoid confusion and mistake, oneshot and classic service use now
the same key name to declare the timeout to use.
TimeoutStart and TimeoutStop are clearer than TimeoutKill and TimeoutFinish.
66 is sufficiently smart to name the file correctly for s6-supervise
whick look for the timeout-kill and timeout-finish file.