gcc 15 and C23 force some union trickery on buffer.h :-(

add a few buffer_init*_forread variants to pretend we have type safety
  make sure buffer_init_staticcontents handles flushing attempts

buffer/buffer_get_token_sa_pred.c

@@ -1,7 +1,5 @@
-#include "byte.h"
 #include "stralloc.h"
 #include "buffer.h"
-#include <errno.h>
 
 int buffer_get_token_sa_pred(buffer* b,stralloc* sa,sa_predicate p) {
   for (;;) {