ebpf: Change error types to c_long

[vadorovsky]

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:

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:

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

The other changes included in this commit are:

• Using core::ffi::c_long instead of aya_cty::c_long, like book examples (they have to be changed too).
• Fixing return values of cgroup_skb, sockopt and sysctl programs (1 means "allow", 0 means "deny").

[tamird]

Looks like a good thing to do. Rebase?

[vadorovsky]

We are inconsistent in usage of c_long and u32 as error codes.

Furthermore, thiserror now supports no_std, so I think it would be better to just use it for all library errors.