Adds `curl-api-file` helper

Instead of having `env=${api_env+.$api_env}` and `api.kind$env` all over
the place, use `kind=$(curl-api-file kind)` to consolidate logic.
Adds `curl-api-var` functions

Makes use of `curl-api`-related files much cleaner.  Also allows
overriding specific vars with `$API_{name}` variables.
set-pulse-server w/ no options removes the setting
Adds password prompt for `curl-api-login`
Adds `-n MAXRETRY` option to `cuke`