zlib: Add support for additional flush mode

[mb@…]

The zlib filter uses Z_SYNC_FLUSH which is the most widely used flush mode. However SSH requires Z_PARTIAL_FLUSH. For further information please see the "Partial Flush" paragraph at: www.bolet.org/~pornin/deflate-flush.html

It would be trivial to add the new flush mode:

File: zlib.hpp

Add
BOOST_IOSTREAMS_DECL extern const int partial_flush;

Add additional argument to zlib_params:

zlib_params(

int flush_mode = zlib::default_flush_mode

add int flush_mode_ member to zlib_base

init flush_mode_ member in zlib_base::do_init

flush_mode_ = p.flush_mode;

and in the filter function change the xinflate to:

int result = xinflate(flush_mode_);

File: zlib.cpp

const int partial_flush = Z_PARTIAL_FLUSH;

const int default_flush_mode = sync_flush;

Please let me know if there is an interest for the diff files.