Changes

icmpv6: Re-license code under GPL-v2-or-later

The original statement in my code from osmo-ggsn commit
d46bcd236e93432c894a939f4e5810dc5e9b4974 in 2017 was worded GPLv2-only,
which was due to the fact that the original OpenGGSN code is v2-only.

All subsequent modifications to this code were done by either me or Pau
Espin, who was doing the work under contract with sysmocom, of which I'm
the managing director.  Therefore, I have legal authority to re-license
this file to v2-or-later.

Change-Id: I5a4fa18ef64c2f228e21fa2c067e55eaa4080508

ip_checksum: Add SPDX license identifier

Let's make sure the newly-created/imported file has a machine-readable
SPDX identifier.

Change-Id: Ic40d01771e8d01461ed81d1a60d9e91a48fdc8d0

Introduce ip_checksum API

ip_checksum.* files are copied from checksum.* in osmo-ggsn.git
1.14.0 (ebd1bf8adbc8d5bec9e36d70e3a78e8838226e7a).

Change-Id: Ia09645a4715ac133036d7c89297b0fa10cf9aa3b

Introduce API osmo_icmpv6

icmpv6.* files are copied from osmo-ggsn.git 1.14.0
(ebd1bf8adbc8d5bec9e36d70e3a78e8838226e7a).

Change-Id: I78ec7270c717af0a1b8ffd9398cd69ea7a0dbee2

stream: Improve error handling and logging in write_cb

Nowhere in man 2 write/send/sendmsg it can be read that a return of 0
is actually an error, so avoid handling that case as error in
stream_cli.

While at ti, log the errno string.

Change-Id: I68468f0452cbc86b6210bbd1dbfa251579270adb