platforms: support symlinking directories to configured locations

[oliver-sanders]

Bullet point (2) "Support moving some directories to different locations with symlinks to the original location" from the platforms proposal document.

Re-implement the root-dir functionality from rose suite-run in Cylc. This allows, for example, the log directory to be placed on a more appropriate filesystem.

Pull requests welcome!

[oliver-sanders]

This issue comes in two parts:

1. The initial creation of the directories on the "localhost" platform when the suite is run.
2. The subsequent creation of these directories on other install targets (filesystems) during the remote-init process when the first job is submitted to a new install target.

[hjoliver]

Note that run-dir symlinking will render cylc register --run-dir=RUNDIR obsolete. The PR for this Issue should remove that option.