How about this: rename the existing readmailcapfile as an internal _readmailcapfile with the new signature.  Then add a backward compatible readmailcapfile with the existing signature/return value that uses a dummy value for lineno and throws away the lineno on output.  The point here is just to avoid breaking programs that are using the existing api, even though it is an internal one.  Yes, it is cruft, but backward compatibility sometimes requires cruft.  We could deprecate the old api with a message that says it is an internal method and should not be used, and see if anyone complains about it being deprecated.