Leaving 'libbpfgo.h' only with declarations to avoid multiple
declaration errors when importing 'libbpfgo.h' from different Go files.

Prefix all C helpers used in Go with 'cgo_', renaming two cases
where the functions does not reflect the libbpf API:

set_print_fn -> cgo_libbpf_set_print_fn
get_internal_map_init_value -> cgo_bpf_map__initial_value

Introduced struct helpers to manage the C struct lifecycle on the C side,
which avoids problems with structs which may contain bitfields. See aquasecurity#244.

  These are replacements for 'BPFMapBatchOpts' and 'bpfMapBatchOptsToC()'.
  - cgo_bpf_map_batch_opts_new
  - cgo_bpf_map_batch_opts_free

  These are replacements for 'bpfMapCreateOptsToC()'
  - cgo_bpf_map_create_opts_new
  - cgo_bpf_map_create_opts_free

Restructured libbpf BPF map logic into separate files:
- 'map-common.go' contains generic BPF map types, constants, and helpers
  like 'GetMapInfoByFD()'.
- 'map-low.go' includes 'BPFMapLow' and respective logic and helpers
  like 'CreateMap()'.
- 'map-iterator.go' contains related types and logic.

In the 'BPFMap' struct:
- Removed fields like 'fd' and 'name' since they must be retrieved
  directly from 'libbpf'.
- Added 'bpfMapLow' instance for access to low-level methods.
- Deprecated 'BPFMap.GetMaxEntries' in favor of using 'MaxEntries'.
- Exposed 'InitialValue' and 'SetInitialValue' as public wrappers.
- Introduced MapFlags(), IfIndex() and MapExtra() methods.

Introduced 'misc.go' as a place for miscellaneous generic helpers.

Bug Fixes:
- Fixed cases where 'BPFMap.ValueSize()', possibly 0, was being passed
  directly to 'make()' - detected on the map of maps effort aquasecurity#343.

Further Refactoring and Standardization on the map related logic and on
lines touched by other changes:
- Removed check for 'BPFMap.Name()' against nil, as 'C.GoString(C.NULL)'
  returns "".
- Changed error returns to 'retC' in functions that return error values
  to avoid confusion with possible use of errno.
- Changed returns to 'valueC' in functions that return values or
  pointers to values.
- Suffixed variable names that are C types with 'C'.
- Applied the use of 'defer' for better resource management.

The `ReuseFD` function links the current BPFMap instance to a provided
map file descriptor. This enables the reuse of a map that was initiated
by another process.

Initial effort: aquasecurity#346

Co-authored-by: Francisco Javier Honduvilla Coto <[email protected]>