features: fix unsupported progtype/helper probes

By enabling feature probes for the program types Tracing, Extension, LSM
and StructOps within the HaveProgramType API, we overlooked that this
would cause false negative results when using these types with the
HaveProgramHelper API. To probe for these program types we craft
specialized ProgramSpecs, but don't do the same when probing for these
types in combination with an arbitrary helper. For now we should return
an inconclusive error from the HaveProgramHelper API until we find a
better way to probe for helpers with these program types.

Fixes: cilium#1082

Signed-off-by: Robin Gögge <r.goegge@isovalent.com>

CI: disable cache

Something broke Go module caching on the semaphore side. Disable
restoring the cache for now.

Signed-off-by: Lorenz Bauer <oss@lmb.io>

internal/unix: add some documentation and tidy up stubs

Document why the package exists, and how stubs will behave on non-Linux OS.
Simplify constants for other OS by giving them a distinct but meaninless value.
Always use type aliases for structs on Linux and remove boilerplate function
comments.

link: implement kprobe_multi link type

As of Linux 5.18, or commit 5a5c11ee3e65 ("Merge branch 'bpf: Add kprobe
multi link'"), attaching multiple k(ret)probes using a single system call
is now possible.

This commit adds support for this through the KprobeMulti() and
KretprobeMulti() APIs in package link.

Co-authored-by: Timo Beckers <timo@isovalent.com>

internal: include previous line on BPF context access error

Add context if the last verifier log line indicates a context access
error.

See cilium#723

features: rename HaveProgType API

In order to be in line with other naming schemes throughout
the library this commit deprecates HaveProgType() in favor
of HaveProgamType().

Signed-off-by: Robin Gögge <r.goegge@gmail.com>

btf: move vmlinux search logic into findVMLinux()

loadKernelSpec() gets short-circuited in the first statement on most kernels,
resulting in the vmlinux search code being rarely exercised successfully in CI.

Split the search logic out into its own function to make it testable in
isolation. Although virtme VMs don't have images at any of these paths,
running the test suite on a host with a full distro does result in a proper
test execution.

Signed-off-by: Timo Beckers <timo@isovalent.com>

Deprecate StructOpts and perf.IsClosed()

These were deprecated and scheduled to be removed in 0.8.0.

Signed-off-by: Timo Beckers <timo@isovalent.com>

README.md: changes for clarity

Signed-off-by: Liz Rice <liz@lizrice.com>

map: include errnos with errors

We replace some errnos from map syscalls with more descriptive
sentinel errors. Include the original errno with the error so
that both

    errors.Is(err, ErrKeyNotExist)
    errors.Is(err, unix.ENOENT)

work. We'll use this for other errors in the future where we
want to introduce our own errors without breaking callers that
rely on the (coarser) errno semantics.