libbpf: Support BPF token path setting through LIBBPF_BPF_TOKEN_PATH envvar

To allow external admin authority to override default BPF FS location
(/sys/fs/bpf) for implicit BPF token creation, teach libbpf to recognize
LIBBPF_BPF_TOKEN_PATH envvar. If it is specified and user application
didn't explicitly specify bpf_token_path option, it will be treated
exactly like bpf_token_path option, overriding default /sys/fs/bpf
location and making BPF token mandatory.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20240124022127.2379740-29-andrii@kernel.org