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And simplify `install` target

- Avoid unreadable spaghetti code
- Avoid to access inexistent message files
    - Which leads to failed command
    - And generates useless error messages
- Generate warning when no message text is found

Also drop evaluation of escape characters in the file

Cycle through files properly, simplify cycle variable and avoid
evaluation of subshell stdout

- The value of `compat` variable was in fact negated
- New (and preffered) way to call the script was not included in usage
- Also generate a warning when running in compatible mode

Rely on PATH env variable

- Compare string rather than execute variables content (which is
  potentially insecure)
- Do not evaluate variables in arithmetic environment

Even though notification with empty message is not the expected kind of
notification and it make no sense to create such notification,
list_notification should export expected json structure and fallback to
empty string in its output.

`exit` is trapped to call unlock anyway

- mail_from is needed in compose_message and create_msmtp_config
- defined turris_smtp comes handy

This reverts commit 01552301.

Let's not go that way yet