runtime: fix program termination when main goroutine calls Goexit
Do not consider idle finalizer/bgsweep/timer goroutines as doing something useful.
We can't simply set isbackground for the whole lifetime of the goroutines,
because when finalizer goroutine calls user function, we do want to consider it
as doing something useful.
This is borken due to timers for quite some time.
With background sweep is become even more broken.
Fixes #7784.
LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/
87960044