libnetwork: Remove not-so-useful printf.

We don't really need to see in syslog when an application is
running in R5 compatibility mode.

libnetwork: Rename find_own_image() to set_own_image_location().

Thanks Axel for the review!

network: Utilize image_id argument to initialize_before.

We don't need to search for our own image_id, we already have it now,
so just use it.

headers/kernel: Include declarations of initialization functions in image.h.

No "real" functional change, but this causes GCC7 to throw errors when
these functions are declared without the image_id argument, which
in some files they were (as this commit repairs.)

This change is largely inconsequential on x86, but on callee-cleanup-args
targets, leaving out the argument would probably cause stack corruption.

libnetwork: Move from src/kits to src/system.

Properly speaking, this is part of POSIX and not of the Be-style
"kits", and so it should live in system/ alongside libroot.
No functional change intended.

Change-Id: I0fcf78a09c76e220ad4f1719d147978ef4a3bc52
Reviewed-on: https://review.haiku-os.org/726
Reviewed-by: Axel Dörfler <axeld@pinc-software.de>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>