Support terminate signals in repository open and write operations ...

SIGINT and SIGTERM are used to terminate ongoing Kopia operations.

kopia snapshot migrate signal handler setup comes late only after setting up the source repository, preventing termination during opening of repositories. In addition, checks for context cancellation during the snapshot copy process are too sparse to effect a termination during some writes.

Consider moving signal handler setup early in command execution. Consider additional checks for context cancelation during repository write.