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_mmapread.c

@@ -1,7 +1,10 @@
 #include <libowfat/buffer.h>
 #include <libowfat/mmap.h>
 
-static ssize_t op() {
+static ssize_t op(int fd,void* buf,size_t l) {
+  (void)fd;
+  (void)buf;
+  (void)l;
   return 0;
 }
 
@@ -9,7 +12,7 @@ int buffer_mmapread(buffer* b,const char* filename) {
   if (!(b->x=(char*)mmap_read(filename,&b->n))) return -1;
   b->p=0; b->a=b->n;
   b->fd=-1;
-  b->op=op;
+  b->op.rop=op;
   b->deinit=buffer_munmap;
   return 0;
 }