conf: cylc executable => cylc path

[oliver-sanders]

sibling: cylc/cylc-doc#206

Small change that came out of comms mode = ssh discussions.

The cylc executable config is a bit strange, for remote cylc invocations the cylc executable is used, however, for job scripts the containing directory is added to the $PATH.

We have had to add a check in multiple places to make sure that the cylc executable isn't set to something which isn't cylc.

This PR:

• Changes cylc executable to cylc path.
• Documents the role of cylc path in the job script.
• Corrects the use login shell docs for the profile files that are sourced.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Already covered by existing tests.
• Appropriate change log entry included.
• (master branch) I have opened a documentation PR at cylc/cylc-doc/pull/XXXX.
• No dependency changes.

[oliver-sanders]

Do we want a changelog for this small global configuration change?

[datamel]

Thanks @oliver-sanders.

[hjoliver]

👍

[hjoliver]

Do we want a changelog for this small global configuration change?

I'll add one on the fly...