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.

examples: update main module

link: return os.ErrNotExist when symbol doesn't exist

Some of the tracing related links currently return ErrNotSupported
when trying to attach to a non-existant symbol. This is misleading.
Consistently return os.ErrNotExist in such cases.

prog: document ProgramSpec's KernelVersion field more accurately

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

program: Support Unpin operation for Program

info: add Name to MapInfo

Allow users to access the map name. The name is empty if the kernel
doesn't support object names yet.

btf: avoid Type copy in FuncProto.walk

FuncProto.walk currently takes a copy of FuncParam.Type since the
for loop implicitly makes a copy of the FuncParam value:

    for _, m := range fp.Params {

m is a copy of the original param, so &m.Type is the address of a
temporary variable. Instead, iterate params using the index and
take the correct address. Also extend the tests to cover this case.