Trial fix for ticket:45

Trial fix for ticket:45

Changed interpretation of second argument to gr_buffer_add_reader from
"history" to "nzero_preload".  If the argument is 0, no zeros are
preloaded into the buffer.  This is a less surprising interpretation
than the old behavior, which when passed a zero, inserted buffersize-1
zeros ;)

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@3403 221aa14e-8319-0410-a670-987f0aec2ac5