libbpf: Rewrite btf datasec names starting from '?'

Optional struct_ops maps are defined using question mark at the start
of the section name, e.g.:

    SEC("?.struct_ops")
    struct test_ops optional_map = { ... };

This commit teaches libbpf to detect if kernel allows '?' prefix
in datasec names, and if it doesn't then to rewrite such names
by replacing '?' with '_', e.g.:

    DATASEC ?.struct_ops -> DATASEC _.struct_ops

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240306104529.6453-13-eddyz87@gmail.com

src/libbpf_internal.h

@@ -374,6 +374,8 @@ enum kern_feature_id {
 	FEAT_UPROBE_MULTI_LINK,
 	/* Kernel supports arg:ctx tag (__arg_ctx) for global subprogs natively */
 	FEAT_ARG_CTX_TAG,
+	/* Kernel supports '?' at the front of datasec names */
+	FEAT_BTF_QMARK_DATASEC,
 	__FEAT_CNT,
 };