[CM T0] Correct the behavior for length plugin

[Gratla]

Expected behavior

• checker plugins should run their validation during ...Get() and ...Set().
• If a validation problem occurs during ...Set(), the plugin should set an error and return -1
• If a validation problem occurs during ...Get(), the plugin should add a warning and continue validation (finally return 0)
• plugins need to have the placements: pregetstorage and postgetstorage in their metadata. Otherwise ...Get()/...Set() won't be called.

The behaviour needs to be fixed in the following file:
https://github.com/ElektraInitiative/libelektra/blob/master/src/plugins/length/length.c

Subissue for #4059.

[mandoway]

I reproduced the issue following the tutorial on the plugin page:

$ sudo apt update
$ sudo apt install libelektra5-experimental

$ sudo kdb mount test.ni /tests/length length ni
$ sudo kdb meta-set spec:/tests/length/text check/length/max 3

// This is expected and correct
$ kdb set user:/tests/length/text asda
Sorry, module length issued the error C03200:
Validation Semantic: Length check of key 'user:/tests/length/text' with value 'asda' failed. Maximum length is 3 but the given string has length 4

$ kdb set user:/tests/length/text asd
Create a new key user:/tests/length/text with string "asd"


// Edit the file to contain a string with more than 3 characters
$ kdb file user:/tests/length/text
/home/markus/.config/test.ni
$ sudo nano /home/markus/.config/test.ni

// This command should actually return a warning
$ kdb get /tests/length/text
asda

[kodebach]

Great to see #4059 is slowly getting done.

[kodebach]

// Edit the file to contain a string with more than 3 characters
$ kdb file user:/tests/length/text
/home/markus/.config/test.ni
$ sudo nano /home/markus/.config/test.ni

A minor thing, but the part above could be replaced with the following to make the snippet work without additional user input:

$ sudo sed -i 's/text = asd/text = 1234/g' $(kdb file user:/tests/length/text)

Also, when posting reproduction snippets you don't need to include the apt update and apt install commands. We'll automatically assume you used the latest release of Elektra.