Skip parsing argv[0] for remap rules

[dhood]

I turned on debug logging investigating #248 and was surprised that argv[0] was being parsed.

$ ros2 run demo_nodes_cpp talker
[WARN] [rcl]: arg 0 (/home/dhood/ros2_ws/install_isolated/demo_nodes_cpp/lib/demo_nodes_cpp/talker) error 'Expected lexeme type (19) not found, at /home/dhood/ros2_ws/src/ros2/rcl/rcl/src/rcl/lexer_lookahead.c:227'
[INFO] [talker]: Publishing: 'Hello World: 1'

(error message format using changes in #248)

Is it safe to assume that argv[0] is never going to have remapping arguments? (I am not 100% sure)

[dhood]

I've found that there are times when this assumption (that argv[0] won't have remapping arguments) doesn't hold: when they are being passed in manually.

E.g. this rclpy test:

self = <test.test_node.TestCreateNode testMethod=test_node_arguments>

    def test_node_arguments(self):
        rclpy.init()
        try:
            node = rclpy.create_node('my_node', namespace='/my_ns', cli_args=['__ns:=/foo/bar'])
>           self.assertEqual('/foo/bar', node.get_namespace())
E           AssertionError: '/foo/bar' != '/my_ns'
E           - /foo/bar
E           + /my_ns

That is a test, yes (and there are a number of other tests relying on the existing behaviour), but I think it's reasonable usage that someone else might want to do.

Unless we think that manual passing of cli_args should always include the process name as the first in the list, I will close this PR.