Return ErrRestrictedKernel when kernel pointers are restricted using sysctls

[dylandreimerink]

info: return ErrRestrictedKernel when program info is restricted

On systems where the `kernel.kptr_restrict` and
`net.core.bpf_jit_harden` sysctls are enabled, certain fields in the
program info may be restricted. When this is the case xlated and jitted
instruction, line info, and function info are unavailable.

When such fields are unavailable, we do get *Len fields set to non-zero
values, but the kernel will have not written any data to the pointers
provided.

At present when this happens we don't recognize this and attempt to
parse the empty buffers resulting in a `parse func info: offset 0:
type ID 0 is a *btf.Void, but expected a Func` error.

The only clue we have that this is happening is that the kernel will
zero out the insns pointer. This commit adds logic to detect when
this happens and then returns a dedicated `ErrRestrictedKernel` error
so this case can be handled gracefully.

This is similar to / based on the approach bpftool has taken to handle this.

kallsyms: return ErrRestrictedKernel when reading zero address

Reading a zero address for an existing kallsyms address is never valid,
so make internal.AssignAddresses return ErrRestrictedKernel if it encounters
a zero address due to kernel.kptr_restrict being set on the host.

To enable users to gracefully fall back to a non-ksym implementation, tolerate
ErrRestrictedKernel during ksym resolution unless there are required ksyms in
the CollectionSpec.