Whitespace detection and removal

[atheriel]

Some internal discussion indicates that "user accidentally copied some whitespace along with the database password" is a common source of errors with existing tooling. It may be sensible for the rskey CLI to detect extra whitespace automatically and remove it.

Some notes:

• Doing this quietly could in theory surprise users, so we may also need to tell them in a sensible way.
• Some secrets (rarely, I'd guess) may actually need to contain leading or trailing whitespace, so we'd need to add a flag to turn this behaviour off.
• A related issue is "user accidentally copied the database password with surrounding " marks included".