Check for invalid listener URLs

[ghjm]

This PR verifies that the given URL for a Receptor peer does not include extraneous information such as a path, and also checks earlier that it matches one of the known schemes. See ansible/receptor#153.

[matburt]

Would be good to have some tests for this, otherwise looks good

[Ichimonji10]

Does this mean that --listen=receptor://127.0.0.1:8888 is now invalid?

[ghjm]

Oops, no, I didn't mean to remove receptor://. I'll fix it.

[ghjm]

Actually, receptor:// still works with this code as-is. The string replace is done before we check the set of allowed schemes, so receptor:// has already been translated to rnp:// and passes the test.