eliminate bpf_progs/netd.h (via cut'n'paste into netd.c)

This file isn't used anywhere else, and the things it contains
should either be in the netd.c implementation or in a different
header file shared with netd's C++ code.

Step one eliminates it, and moves its content into the .c program.

Test: builds, atest
Bug: 146787904
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I53348bda8c8e84210735a3324fbb6f48396c8427