With this commit we add a new subpackage `features`. `features`
provides an API for calling processes to probe the kernel for available
BPF features. The first public API function added with this commit is
`HaveMapType(mt ebpf.MapType) error`, which allows to probe for
available BPF map types. The results of an API call are stored in an
internal cache, resulting in a probe being run at most once even if
called repeatedly.

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

- Removed naked error comparison to ErrNotSupported (although valid)
- Converted error handling logic to switch statement so only a single case may fire.
  Before, it was possible to hit multiple cases if the error from BPFMapCreate
  contained multiple wrapped errors.
- Skipped wrapping of EPERM to avoid ending up with 'unexpected error during feature probe:
  operation not permitted'. Permission errors are not unexpected.

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

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