Support reload in falco systemd units

[jabdr]

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

/kind release

Any specific area of the project related to this PR?

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/area build

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What this PR does / why we need it:
Support easy reload with systemd units

Which issue(s) this PR fixes:

Fixes #2587

Special notes for your reviewer:

Does this PR introduce a user-facing change?:
YES

new: support systemctl reload for Falco services

[leogr]

/milestone 0.36.0

[FedeDP]

Thank you for this! Since we are very close to the 0.35.0 release, we target this to the 0.36 release!

[Andreagit97]

we will take a look soon!

[FedeDP]

ExecReload=

    Commands to execute to trigger a configuration reload in the service. This argument takes multiple command lines, following the same scheme as described for ExecStart= above. Use of this setting is optional. Specifier and environment variable substitution is supported here following the same scheme as for ExecStart=.

    One additional, special environment variable is set: if known, $MAINPID is set to the main process of the daemon, and may be used for command lines like the following:

    ExecReload=kill -HUP $MAINPID

Given the system unit documentation, this seems correct and a nice improvement.
@jabdr Can you rebase on top of master?
Then we will quickly test the feature :)

[Andreagit97]

Rebased it :)

[leogr]

nice 👍

[poiana]

LGTM label has been added.

Git tree hash: 548e35c20cc932db7210a247f6da6b8834fc38e6

[Andreagit97]

/hold

[Andreagit97]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Andreagit97, jabdr, leogr

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