Honor FORCE_COLOR when terminfo has no setaf

[am11]

WriteSetColorString defers entirely to terminfo, so
Console.ForegroundColor writes nothing when the terminal offers no
setaf/setab, even with FORCE_COLOR=1.

env -u TERM FORCE_COLOR=1 ./app    # no escapes
FORCE_COLOR=1 ./app                # escapes

Fall back to standard SGR sequences only when color was explicitly
forced; when it is on merely because output is not redirected, an
absent capability still means no color. ResetColor gets a matching
ESC[0m.

This also overrides TERM=dumb, since FORCE_COLOR is the later and
equally explicit signal.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-console
See info in area-owners.md if you want to be subscribed.