MINOR: buffer: make b_getblk_nc() take const pointers

MINOR: buffer: make b_getblk_nc() take const pointers

Now that there are no more users requiring to modify the buffer anymore,
switch these ones to const char and const buffer. This will make it more
obvious next time send functions are tempted to modify the buffer's output
count. Minor adaptations were necessary at a few call places which were
using char due to the function's previous prototype.