Co-authored-by: Frantisek Sumsal <frantisek@sumsal.cz>

IPPROTO_L2TP was moved from linux/l2tp.h to linux/in.h [0], so let's
reflect that change to fix build with newer kernels:

```
In file included from ../src/libsystemd/sd-netlink/netlink-types-genl.c:10:
../src/basic/linux/l2tp.h:16: error: "IPPROTO_L2TP" redefined [-Werror]
   16 | #define IPPROTO_L2TP            115
      |
In file included from ../src/libsystemd/sd-netlink/netlink-types-genl.c:3:
/usr/include/netinet/in.h:85: note: this is the location of the previous definition
   85 | #define IPPROTO_L2TP            IPPROTO_L2TP
      |
cc1: all warnings being treated as errors
```

When at it, update the rest of the headers we ship as well.

[0] https://github.com/torvalds/linux/commit/65b32f801bfbc54dc98144a6ec26082b59d131ee

loadavg.h is an internal header of the Linux source repository, and as
such it is licensed as GPLv2-only, without syscall exception.

We use it only for 4 macros, which are simply doing some math calculations
that cannot thus be subject to copyright.
Reimplement the same calculations in another internal header and delete
loadavg.h from our tree.

As far as I can see, we use this to get a list of ARPHRD_* defines (used in
particular for Type= in .link files). If we drop our copy, and build against
old kernel headers, the user will have a shorter list of types available.  This
seems OK, and I don't think it's worth carrying our own version of this file
just to have newest possible entries.

7c5b9952c4f6e2b72f90edbe439982528b7cf223 recently updated this file, but we'd
have to update it every time the kernel adds new entries. But if we look at
the failure carefully:

src/basic/arphrd-from-name.gperf:65:16: error: ‘ARPHRD_MCTP’ undeclared (first use in this function); did you mean ‘ARPHRD_FCPP’?
   65 | MCTP, ARPHRD_MCTP
      |                ^~
      |                ARPHRD_FCPP

we see that the list we were generating was from the system headers, so it was
only as good as the system headers anyway, without the newer entries in our
bundled copy, if there were any. So let's make things simpler by always using
system headers.

And if somebody wants to fix things so that we always have the newest list,
then we should just generate and store the converted list, not the full header.

ARPHRD_MCTP was added in 5.14. Sync if_arp.h to pick up the definition

Fixes #20694