libbpf: Rename DECLARE_LIBBPF_OPTS into LIBBPF_OPTS

It's confusing that libbpf-provided helper macro doesn't start with
LIBBPF. Also "declare" vs "define" is confusing terminology, I can never
remember and always have to look up previous examples.

Bypass both issues by renaming DECLARE_LIBBPF_OPTS into a short and
clean LIBBPF_OPTS. To avoid breaking existing code, provide:

  #define DECLARE_LIBBPF_OPTS LIBBPF_OPTS

in libbpf_legacy.h. We can decide later if we ever want to remove it or
we'll keep it forever because it doesn't add any maintainability burden.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/bpf/20211103220845.2676888-2-andrii@kernel.org

tools/lib/bpf/libbpf_common.h

@@ -54,7 +54,7 @@
  * including any extra padding, it with memset() and then assigns initial
  * values provided by users in struct initializer-syntax as varargs.
  */
-#define DECLARE_LIBBPF_OPTS(TYPE, NAME, ...)				    \
+#define LIBBPF_OPTS(TYPE, NAME, ...)					    \
 	struct TYPE NAME = ({ 						    \
 		memset(&NAME, 0, sizeof(struct TYPE));			    \
 		(struct TYPE) {						    \