libbpf: make libbpf_errno.c into more generic libbpf_utils.c

Libbpf is missing one convenient place to put common "utils"-like code
that is generic and usable from multiple places. Use libbpf_errno.c as
the base for more generic libbpf_utils.c.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20251001171326.3883055-2-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>