Follow up to https://bugzilla.samba.org/show_bug.cgi?id=14983

Currently there are lists of errnos in the libsmbclient header. These lists look exhaustive (but are not necessarily - bug #15088), they may go out of date as internals change (cause it's hard to keep track) and they are also not necessarily attached to a single meaning (e.g. EINVAL from bug #14983 may point at a credential error but supposedly also other invalid inputs).

Two solutions come to mind:

a) Put a note somewhere at the top "Errno listings are meant as guidance and neither exhaustive listings nor exhaustive in meaning. Implementations must be prepared to attempt error handling with any error code."

b) Prefix the @return format:

> * @return          Returns 0 on succes. Returns 1 on failure with errno set.
> *                  Errno may be and may mean:
> *                  - EBUSY Server connections are still used, Files are open or cache
> *                          could not be purged
> *                  - EBADF context == NULL

I'd go with b), it's more noise but also super obvious.

I can prepare a diff if we pick an option. Or other ideas come up, I'm open to everything :)