Create new process group for pgcopydb initialization. (#211)

Previously, when `pgcopydb` encountered a FATAL error and called
`copydb_fatal_exit`, any wrapper processes around `pgcopydb` would also get
terminated due to the `if (kill(0, SIGTERM) == -1)` statement. To fix this
issue, a new process group is now created for `pgcopydb` during
initialization. This ensures that only the `pgcopydb` process will be
terminated in case of a FATAL error, leaving the wrapper processes intact.