Allow using a new process group for the terraform cli child process

[kristina-solovyova]

Support using syscall.SysProcAttr with Setpgid: true on the Cmd struct, which creates a new process group for the child process.
This effectively separates the child process from the parent process's signal group, so it won't receive the interrupt signal when you interrupt the parent process.
Motivation - graceful shutdown implementation. When receiving CTRL+C the child process running terraform is interrupted, which is undesirable for my use case.

See: https://stackoverflow.com/questions/33165530/prevent-ctrlc-from-interrupting-exec-command-in-golang/33171307#33171307

[hashicorp-cla]

All committers have signed the CLA.

[spirit1431007]

How about Windows, we can use CREATE_NEW_PROCESS_GROUP