btf: fix some split BTF shortcomings

[lmb]

Fix some outright bugs and some annoyances with split BTF support. It's still not entirely great, but this is better than nothing.

Requirement for #859.

[lmb]

API changes:

--- old.txt	2022-11-20 16:21:35.946436516 +0000
+++ new.txt	2022-11-20 16:21:29.014452875 +0000
@@ -21,6 +21,9 @@

 FUNCTIONS

+func ClearCachedKernelSpec()
+    ClearCachedKernelSpec flushes any cached kernel type information.
+
 func LoadSpecAndExtInfosFromReader(rd io.ReaderAt) (*Spec, *ExtInfos, error)
     LoadSpecAndExtInfosFromReader reads from an ELF.

@@ -270,11 +273,9 @@
 func (h *Handle) Info() (*HandleInfo, error)
     Info returns metadata about the handle.

-func (h *Handle) Spec(base *Spec) (*Spec, error)
+func (h *Handle) Spec() (*Spec, error)
     Spec parses the kernel BTF into Go types.

-    base is used to decode split BTF and may be nil.
-
 type HandleInfo struct {
 	// ID of this handle in the kernel. The ID is only valid as long as the
 	// associated handle is kept alive.

[ti-mo]

Ack overall, but this needs to take the changes of #855 into account, I think there's some overlap with btf_spec.go:vmlinux{,Spec}.

[ti-mo]

Thanks! Would like some clarification around the changes to TypeByID.