Set proper cache dir for posix build

[toots]

This PR enhances caching support by separating system vs. user cache.

System cache is mostly for stdlib caching and should be shared by all users while user cache is for user scripts and should be user-specific.

Default locations:

• windows: <executable directory>/cache for both
• posix (system-wide install):
  • system: /var/cache/liquidsoap
  • user: $HOME/.cache/liquidsoap
• dune:
  • system: set by dune
    • Local build: _build/install/default/share/liquidsoap-lang/cache
    • Installed: ~/.opam/<switch>/share/liquidsoap-lang/cache
  • user: $HOME/.cache/liquidsoap

Fixes: #3970

[toots]

@vitoyucepi Description and PR updated. This should cover all bases now.

[vitoyucepi]

I didn't ask this question before, will the cache contain the hardcoded passwords?

password = "pass"
output.icecast(%mp3, blank(), password=password, mount="1")
output.icecast(%mp3, blank(), password="pass", mount="2")

[toots]

I didn't ask this question before, will the cache contain the hardcoded passwords?

password = "pass"
output.icecast(%mp3, blank(), password=password, mount="1")
output.icecast(%mp3, blank(), password="pass", mount="2")

Excellent catch, thank you.

I just added proper permissions with customization env variables and documented them and also security concerns in the doc.

[toots]

I think that this is good for merge. The docker build issues are related to debian-multimedia changing their DNS I believe.