Add async cli_pull support

Add async cli_pull support

This is the big (and potentially controversial) one. It took a phone call to
explain to metze what is going on inside cli_pull_read_done, but I would really
like everybody to understand this function. It is a very good and reasonably
complex example of async programming. If we want more asynchronism in s3, this
is what we will have to deal with :-)

Make use of it in the smbclient "get" command.

Volker