Forward signals to exec subprocess groups

[puneetdixit200]

Summary

• run foreground exec-env and exec-file subprocesses in a new Unix process group
• forward SIGHUP, SIGINT, SIGTERM, and SIGQUIT to the child process group while waiting for it to exit
• keep Windows behavior unchanged and add a Unix regression test for SIGTERM forwarding

Closes #840

Testing

• GOCACHE=/tmp/sops-840-gocache GOMODCACHE=/tmp/sops-840-gomodcache go test ./cmd/sops/subcommand/exec
• GOCACHE=/tmp/sops-840-gocache GOMODCACHE=/tmp/sops-840-gomodcache go vet ./...
• GOCACHE=/tmp/sops-840-gocache GOMODCACHE=/tmp/sops-840-gomodcache go test $(go list ./... | rg -v '/(hcvault|kms|pgp)$')
• GOCACHE=/tmp/sops-840-gocache GOMODCACHE=/tmp/sops-840-gomodcache go test ./... fails locally because Docker is not available for hcvault/kms tests and gpg is not installed for pgp tests