Check for OS and root privileges in NewDaemon

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Move TMPDIR symlink cleanup to NewDaemon

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Move canonical root path detection to NewDaemon

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Move kernel version/capabilities detection to NewDaemon

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #7343 from shykes/cleanup-main

Cleanup main function