accept_filter: return different errors for non-listener and a busy socket

The fact that an accept filter needs to be cleared first before setting to
a different one isn't properly documented. The requirement that the
socket needs already be listening, although trivial, isn't documented
either. At least return a more meaningful error than EINVAL for an
existing filter. Cover this with a test case.

accf_tls: accept filter that waits for TLS handshake header

tests/socket: add accept_filter(9) test

Test basic functionality of accf_data(9) and accf_http(9)