udev: predictable names for auxiliary sub-function (SF) devices (#42154)

Some drivers (currently mlx5_core) expose sub-functions (SFs) of a PCI
Physical or Virtual Function as auxiliary devices that carry a stable
`sfnum` sysfs attribute (the user-defined SF number passed to `devlink
port add ... sfnum N`). Their leaf devices (net, infiniband, ...)
currently inherit the parent PF/VF's `ID_PATH` and the SF netdev falls
through to the kernel-assigned `eth<N>` name.

Two patches:

1. `udev-builtin-path_id`: prepend an `sf-<N>` token when the walk
crosses an aux device with `sfnum`, so SF leaf devices get an `ID_PATH`
distinct from the parent PF/VF (e.g. `pci-0000:c1:00.0-sf-88`). Aux
devices without `sfnum` keep the pre-patch behaviour, so existing
`ID_PATH` values are unchanged and the patch needs no naming-scheme gate
(path_id is unversioned).

2. `udev-builtin-net_id`: name SF host netdevs analogously to SR-IOV VF
host netdevs — walk to the parent PCI function and append a
single-character `S<sfnum>` suffix. SFs hosted on SR-IOV VFs (VF-SF) get
both suffixes chained on the PF's base name (`enp193s0f0v0S88`). Gated
behind `NAMING_SUBFUNC` / `NAMING_V261`. Man page
(`systemd.net-naming-scheme(7)`) updated.

The patches apply to any driver that exposes its SF leaf devices below
an aux device exposing `sfnum`.

Validated on mlx5 hardware: PF, VF, SF-on-PF (`enp8s0f0S88`,
`pci-0000:08:00.0-sf-88`). VF-SF case verified with a fake-sysfs setup
since this is not supported by any real device and is more a theoretical
use case.

diff --cc NEWS
Simple merge