o Two new public functions: y4m_clear_stream_info(), y4m_clear_frame_info().
o y4m_read_*_header() call these functions (respectively) at beginning.
o y4m_init_*_info() use these functions to set/init the default (non-)values.
Also:
o 'framelength' field in stream_info properly initialized.
o Moved y4m_fi_xtags() around to beautify source code.
