On Windows, the `os.Process.Signal` method returns an error when sending
`os.Interrupt` (SIGINT) because it isn't implemented. This causes test servers
in the `testutil` packages to break on Windows. Use the platform specific
syscalls to generate the SIGINT instead.
The agent's signal handler also did not correctly handle the Ctrl-C because we
were masking os.Interrupt instead of SIGINT.
Fixes: https://github.com/hashicorp/nomad/issues/26775
Co-authored-by: Chris Roberts <croberts@hashicorp.com>
