Add streams support

Add streams support

This is the core of the streams support. The main change is that in
files_struct there is now a base_fsp pointer that holds the main file open
while a stream is open. This is necessary to get the rather strange delete
semantics right: You can't delete the main file while a stream is open without
FILE_SHARE_DELETE, and while a stream is open a successful unlink of the main
file leads to DELETE_PENDING for all further access on the main file or any
stream.
(This used to be commit 6022873cc155bdbbd3fb620689715f07a24d6ed1)