acct(2): begin the deprecation of legacy BSD process accounting

acct(2): begin the deprecation of legacy BSD process accounting

As Christian points out [1], even though it's privileged, this interface
has a lot of footguns. There are better options these days (e.g. eBPF),
so it would be good to start discouraging its use and mark it as
deprecated.

[1]: https://lore.kernel.org/linux-fsdevel/20250212-giert-spannend-8893f1eaba7d@brauner/

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260106-bsd-acct-v1-1-d15564b52c83@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>