Make PBSZ optional about proftpd

[Castaglia]

Thanks for the pull request! I may change the TLSOption name to something else later, e.g. TLSOption RFCConformance or somesuch, some name that might be slightly more human-readable, since some users/admins may not be aware of the PBSZ and its role, specifically. But the functionality will stay the same.

[gczuczy]

You are most welcome, I've fixed the things you've pointed out in the pull request, please take a look at it again.

I've chosen "RequirePBSZ", because it's specifically about that command, and the error message also mentions this command in reply to PROT, when forgotten. There are a rather large number of relevant RFCs, and even RFC2228 has multiple requirements, that's one of the reasons why I chosen a name for the option with "RFC" in it. Personally I think, that might be misleading and confusing on the long run.

[Castaglia]

In the case of FTP over SSL/TLS, there's only one particularly relevant RFC: RFC 4127. However, your point is taken.

Another approach might be to not provide such a TLSOption at all, and simply always treat PBSZ as optional. A quick look through the vsftpd and pure-ftpd source code bases show that those FTP daemons do not enforce a requirement of PBSZ, nor do they provide configurations for requiring them, which lends credence to the idea that the admins running those servers have no need to require a PBSZ command.

[gczuczy]

Honestly, I don't really see any reason why couldn't be optional by default, except if some sysadmin is feeling to start a holy crusade against broken clients, and wants to teach the world a lesson. Apart from that, I don't think it would hurt anyone.

Would you like me to change the pull request to make this the default behaviour?

[Castaglia]

Yes, please. It's easy enough to add such an option in, later, if/when we find a use case.

[gczuczy]

Done, please review. Instead of removing the error return, i've just disabled the codepath with a macro, if that's fine. Might come handy if there's a usecase.

[Castaglia]

PR merged to master; thanks!