Added i_stream_create_seekable_path()

Added istreams_merge(), which forces istream-seekable to be created.

Marked functions parameters that are allowed to be NULL. Some APIs were also changed. The non-obvious APIs where NULL parameter was changed to "" are master_service_init() and auth_master_user_list_init(). These checks can currently be enabled only on a patched clang: http://llvm.org/bugs/show_bug.cgi?id=6786

istream-seekable: Changed API to use a callback function to create the temp file. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Removed memory pool parameter from iostreams. Default pool was almost always used, and the stream usually required destroying anyway so it didn't even make freeing memory easier. --HG-- branch : HEAD

Added istream-seekable, which allows combining multiple input streams and creating a single seekable stream out of them. --HG-- branch : HEAD