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_init_read_allocbuf.c

@@ -2,6 +2,6 @@
 #include <libowfat/buffer.h>
 
 int buffer_init_read_allocbuf(buffer* b, int fd, size_t ylen) {
-  return buffer_init_allocbuf(b, read, fd, ylen);
+  return buffer_init_allocbuf_forread(b, read, fd, ylen);
 }