MEDIUM: http: replace get_ip_from_hdr2() with http_get_hdr()

MEDIUM: http: replace get_ip_from_hdr2() with http_get_hdr()

The new function does not return IP addresses but header values instead,
so that the caller is free to make what it want of them. The conversion
is not quite clean yet, as the previous test which considered that address
0.0.0.0 meant "no address" is still used. A different IP parsing function
should be used to take this into account.