Change zap_buffer_read_loop() to always try to read the full amount of bytes we wante...

Change zap_buffer_read_loop() to always try to read the full amount of bytes we wanted to get; add zap_buffer_seek() to skip n bytes from the beginning of the buffer (zap_buffer_toss works relative to the current position only)

git-svn-id: http://svn.openzap.org/svn/openzap/trunk@539 a93c3328-9c30-0410-af19-c9cd2b2d52af