avio: avio: avio_ prefixes for put_* functions

In the name of consistency:
put_byte           -> avio_w8
put_<type>         -> avio_w<type>
put_buffer         -> avio_write

put_nbyte will be made private
put_tag will be merged with avio_put_str

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
(cherry picked from commit 77eb5504)