feat: switch to a network namespace on Linux

* Add support to run inside Linux namespace

**Motivation:**
In a specific use case, we use sshuttle to provide access to private
networks from multiple sites to a specific host. The sites may contain
networks that overlap each other, so each site is accessed inside a
different namespace that provides process-level network isolation and
prevents network overlap.

**Objective:**
This commit just adds a convenient way of spawning multiple sshuttle
instances inside different namespaces from a single process, by passing
the namespace's name though the variable --namespace. The result is the
same as calling `ip netns exec $NAMESPACE sshuttle ...`

* Add the argument --namespace-pid

The argument '--namespace-pid' allows sshuttle to attach to the same net
namespace used by a running process.

* PEP-8 compliance

* Add comment

* Make --namespace and --namespace-pid mutually exclusive.

* Prevent UnicodeDecodeError parsing iptables rule with comments

If one or more iptables rule contains a comment with a non-unicode character, an UnicodeDecodeError would be raised.

tests/client/test_options.py

@@ -176,3 +176,33 @@ def test_parse_subnetport_host_with_port(mock_getaddrinfo):
             (socket.AF_INET6, '2404:6800:4004:821::2001', 128, 80, 90),
             (socket.AF_INET, '142.251.42.129', 32, 80, 90),
         ])
+
+
+def test_parse_namespace():
+    valid_namespaces = [
+        'my_namespace',
+        'my.namespace',
+        'my_namespace_with_underscore',
+        'MyNamespace',
+        '@my_namespace',
+        'my.long_namespace.with.multiple.dots',
+        '@my.long_namespace.with.multiple.dots',
+        'my.Namespace.With.Mixed.Case',
+    ]
+
+    for namespace in valid_namespaces:
+        assert sshuttle.options.parse_namespace(namespace) == namespace
+
+    invalid_namespaces = [
+        '',
+        '123namespace',
+        'my-namespace',
+        'my_namespace!',
+        '.my_namespace',
+        'my_namespace.',
+        'my..namespace',
+    ]
+
+    for namespace in invalid_namespaces:
+        with pytest.raises(Fatal, match="'.*' is not a valid namespace name."):
+            sshuttle.options.parse_namespace(namespace)