Non-functional improvements of the Jail helpers

[Ashod]

Non-functional changes. Non-disruptive to allow cherry-picking to release branches with minimal effort (no large code movements, for example).

Each commit is self contained and has more explanation in the commit-message.

• wsd: jail: rename jails to childroot to distinguish them
• wsd: jail: move LO_JAIL_SUBPATH to JailUtil
• wsd: jail: no need to pass loSubPath between processes
• wsd: jail: add Poco namespace where missing in LinkOrCopy
• wsd: jail: encapsulate LinkOrCopy into a class
• wsd: jail: unlink before copying files
• wsd: jail: simplify linkOrCopy member names

[mmeeks]

Hi Ash; soo .... this has been lingering for over a year - I wonder if we should (when there is time after the 21.11 release) - re-visit these if they're still relevant (?) or should we close this ? Thanks ! =)

[mmeeks]

Ash - should we re-visit or close for 24.04 ? I'm minded to close for now, and re-open and rescue if you'd like to bring some of this back =)