cleanup: Modernize C++ struct/enum/union declarations

[federico-sysdig]

What type of PR is this?
/kind cleanup

Any specific area of the project related to this PR?

Does this PR require a change in the driver versions?
no

What this PR does / why we need it:
From the falcosecurity/libs project's coding guidelines:

Also we think that defining the struct as a typedef makes forward declarations
clunky and find using the C++ style when declaring our structs makes our
lives easier.

   //
   // Us human parsers find this confusing.
   //
   typedef struct _my_struct
   {
     u_int16	m_field;
   } my_struct,
   *p_my_struct;

   //
   // This is easier!
   //
   struct my_struct {
     u_int16	m_field;
   };

This PR changes struct/union/enum definitions to follow just that.
Also, it changes declarations of objects of struct/union/enum type as to remove the struct/union/enum in the C++ code as user-defined types are "first-class citizens" in C++ and don't need a special, longer, syntax.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
The changes are very many, but consider that they are all instances of the same change. For instance struct sinsp_chisel* ch; becomes sinsp_chisel* ch;. Apart the mechanical change, there is no difficult logic to be reviewed in this PR.

Does this PR introduce a user-facing change?:

NONE

[incertum]

Same, could we get a rebase, thanks!

[federico-sysdig]

Same, could we get a rebase, thanks!

@incertum Thanks for the nudge. Done.

[incertum]

/approve

[poiana]

LGTM label has been added.

Git tree hash: f2ce7af701eaaa3883a23a844e7293bc31643960

[incertum]

/approve

[poiana]

LGTM label has been added.

Git tree hash: f2a0e4a125439bb5ee2fe0fbf7bd2b33ae83586c

[Andreagit97]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Andreagit97, federico-sysdig, incertum

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