[vchkpw] Stability of error codes from vchkpw

[Charles Butcher]

I am writing an SMTP authentication plugin for qpsmtpd which will call 
vchkpw.

Different error codes require different treatment; e.g. "incorrect 
password" is fatal, whereas some errors should probably allow a retry 
before dropping the connection.

So can the various error code values from vchkpw be relied upon, or are 
they subject to change?
Are they officially published somewhere in the doco/specs or is the 
source code the only authoritative reference?