Leverage posix builtins + grep (with regex matching) to emit quoted
environment variables to K3S_FILE_ENV, e.g.:

```shell
$ export CONTAINERD_TEST_1='one!1' K3S_TEST_2=two K3S_TEST_3='thr ee' TEST_4='!@#$%^&*()_+four!'
$ sh -c export | while read x v; do echo $v; done | grep -E '^(K3S|CONTAINERD)_'
CONTAINERD_TEST_1='one!1'
K3S_TEST_2='two'
K3S_TEST_3='thr ee'
```

Fixes #3395
Addresses rancher/k3os#708
Signed-off-by: Jacob Blain Christen <jacob@rancher.com>