selftests/bpf: Generalize helpers to control background listener

selftests/bpf: Generalize helpers to control background listener

Move the following routines that let us start a background listener
thread and connect to a server by fd to the test_prog:
* start_server - socket+bind+listen
* connect_to_fd - connect to the server identified by fd

These will be used in the next commit.

Also, extend these helpers to support AF_INET6 and accept the family
as an argument.

v5:
* drop pthread.h (Martin KaFai Lau)
* add SO_SNDTIMEO (Martin KaFai Lau)

v4:
* export extra helper to start server without a thread (Martin KaFai Lau)
* tcp_rtt is no longer starting background thread (Martin KaFai Lau)

v2:
* put helpers into network_helpers.c (Andrii Nakryiko)

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200508174611.228805-2-sdf@google.com