The `helpers` member of the struct is a `string_list` of helpers. Each
string specifies an external helper which will be run, in order, to
either acquire or store credentials. See the section on credential
+helpers below. This list is filled-in by the API functions
+according to the corresponding configuration variables before
+consulting helpers, so there usually is no need for a caller to
+modify the helpers field at all.
This struct should always be initialized with `CREDENTIAL_INIT` or