[part 4/n] Split scap_*.c into platform-specific files

[gnosek]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

/kind feature

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area API-version

/area build

/area CI

/area driver-kmod

/area driver-bpf

/area driver-modern-bpf

/area libscap-engine-bpf

/area libscap-engine-gvisor

/area libscap-engine-kmod

/area libscap-engine-modern-bpf

/area libscap-engine-nodriver

/area libscap-engine-noop

/area libscap-engine-source-plugin

/area libscap-engine-savefile

/area libscap-engine-udig

/area libscap

/area libpman

/area libsinsp

/area tests

/area proposals

Does this PR require a change in the driver versions?

/version driver-API-version-major

/version driver-API-version-minor

/version driver-API-version-patch

/version driver-SCHEMA-version-major

/version driver-SCHEMA-version-minor

/version driver-SCHEMA-version-patch

What this PR does / why we need it:

This PR splits scap_procs.c, scap_fds.c, scap_userlist.c, and scap_iflist.c into platform-independent and platform-specific parts. There are no code changes, except for strerror which now lives in a separate file (scap_procs.c was pretty arbitrary TBH). This means we're dealing with moderately pointless code, like win32/scap_userlist.c but this PR deliberately doesn't address that so it's easier to review.

Git history should be preserved in all the new files.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[FedeDP]

Isn't this a 4/n? :P https://github.com/falcosecurity/libs/pull/487 :rofl:

[gnosek]

One, two, many, lots ;)

[FedeDP]

/milestone 0.10.0

[poiana]

LGTM label has been added.

Git tree hash: 4d71a82bcbd1fcec0b2dafee3e7b5df40dff35ac

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: FedeDP, gnosek, LucaGuerra

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