MINOR: buffer: replace b{i,o}_put* with b_put*

MINOR: buffer: replace b{i,o}_put* with b_put*

The two variants now do exactly the same (appending at the tail of the
buffer) so let's not keep the distinction between these classes of
functions and have generic ones for this. It's also worth noting that
b{i,o}_putchk() wasn't used at all and was removed.