Write asciiz string to buffer
#include <buffer.h>
int buffer_puts(buffer* b,const char* x);
buffer_puts is like buffer_put with len determined as the number of bytes before the first \0 in x.
The difference to buffer_putsalign is that, when there isn't enough space for new data, buffer_puts calls buffer_flush before copying any data, while buffer_putsalign fills all available space with data before calling buffer_flush.
buffer_puts returns 0 if everything was fine, -1 on error (setting errno).