React to SIGTERM exit code change

[Tratcher]

dotnet/coreclr#21300 (comment)

CoreClr is changing the default behavior for SIGTERM exit codes from zero to non-zero. WebHost and GenericHost both block the ProcessExit event while attempting to shut down.

Proposal:
If the host does shut down gracefully then set the exit code to zero.

[tmds]

This change is in coreclr for 3.0. So maybe this should be 3.0 milestone?

[Tratcher]

Note for generic host: We're going to have to port some of the Microsoft.AspNetCore.Server.IntegrationTesting infrastructure to Extensions to enable proper functional tests.