ebpf: Change error types to c_long

All helper functions and context methods are returning `c_long` as an
error. Before this change, aya-template and book examples were adjusting
the error type in `try_*` functionns return type of the program. For
example, for XDP programs programs, which return `u32`, the type
returned by `try_*` functions, was `Result<u32, u32>`.

This approach is forcing people to write boilerplate code for casting
errors from `c_long` to the expected type, like:

```rust
MY_MAP.insert(k, v, 0).map_err(|_| 1u32)?;
```

This change solves that problem by:

* Using `Result<i32, c_long>` as a type for `try_*` functions for
  SKB-based programs and then replacing errors with `TC_ACT_SHOT` or
  `0` once in the main program function.
* Using `Result<u32, c_long>` as a type for `try_*` functions in `try_*`
  funnctions for XDP programs and then replacing errors with `XDP_ABORT`.
* Using either `Result<u32, c_long>` or `Result<i32, c_long>` as types
  in `try_*` functions for all other program types where the return value
  matters (LSM, sysctl etc.) and replacing the error either with `0` or
  `1` (depending on which number means "block the action" for particular
  type of program).
* Using `Result<(), c_long` for all other program types where the
  return value doesn't matter (kprobe, uprobe etc.).

So errors can be handled without casting, like:

```rust
MY_MAP.insert(k, v, 0)?;
```

The other change is fixing return values of cgroup_skb, sockopt and
sysctl programs (1 means "allow", 0 means "deny").

Signed-off-by: Michal Rostecki <[email protected]>