sessiond: lttng: Add command to check kernel tracer status

sessiond: lttng: Add command to check kernel tracer status

Issue observed
--------------

When `lttng enable-channel --kernel` fails, little feedback is
available to user to help them to understand the cause.

Eg.

```
Error: Channel asdf: Kernel tracer not available (session auto-20231123-092621)
```

Solution
--------

The semantic status of the kernel tracer is tracked and persisted in
the session daemon (through `init_kernel_tracer` and
`cleanup_tracer_tracer`.

A new client command `lttng_kernel_tracer_status` is added to request
the current value of the `kernel_tracer_status`. The `lttng` client
uses this command after enabling a kernel-domain channel fails to
provide the user with a more specific cause of the failure.

Eg.

```
Error: Channel asdf: Kernel tracer not available (session auto-20231123-092621)
        Missing one or more required kernel modules
        Consult lttng-sessiond logs for more information
```

The kernel tracer status is tracked with an enum defined in
`include/lttng/kernel.h` to avoid passing potentially different errno values
or locale-dependant strings between the LTTng client and session
daemon.

Loading modules and checking signatures can fail with a number of
different errno values. For example:

C.f. https://gitlab.com/linux-kernel/stable/-/blob/master/kernel/module/signing.c#L70

 * `EKEYREJECTED`
 * Any other error code

C.f. https://gitlab.com/linux-kernel/stable/-/blob/master/Documentation/security/keys/core.rst

 * `EKEYREVOKED`
 * `EKEYEXPIRED`
 * `ENOKEY`
 * Others, such as `ENOMEM`

Known drawbacks
---------------

None.

Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I2ae4b188f0110a472200c2511439b9e3e600527d