FTPClientSession::createDirectory() to return std::string

[stu003]

From RFC 959:

^''''Unfortunately, the argument to MKD may not always be a suitable argument for CWD. This is the case, for example, when a TOPS-20 subdirectory is created by giving just the subdirectory name. That is, with a TOPS-20 server FTP, the command sequence
MKD MYDIR
CWD MYDIR

will fail. The new directory may only be referred to by its 'absolute' name; e.g., if the MKD command above were issued while connected to the directory , the new subdirectory could only be referred to by the name .''''^

And then:

^''"To solve these problems, upon successful completion of an MKD command, the server should return a line of the form:

257 space " directory-name " space commentary

That is, the server will tell the user what string to use when referring to the created directory."''^

So I think it would be much more convenient if FTPClientSession::createDirectory() could return a string with a full path to the created directory that is returned with a server response.