Clean up and improve retreat condition handling

Clean up and improve retreat condition handling

Change retreat condition prompt to point to help.  Before, it listed
conditions rather cryptically, without mentioning how to get help.

Don't provide help when encountering a bad retreat condition
character.

Fail the command when encountering a bad retreat condition character.
Before, they were dropped.

Don't fail the command when the player asks for help at the condition
code prompt.

Retreat condition help failed to explain 'c'.