add option --iobuf (file r/w buffersize):

the default (256 KiB) appears optimal in the most popular scenario
(linux host with storage on local physical disk, usually NVMe)

was previously a mix of 64 and 512 KiB;
now the same value is enforced everywhere

download-as-tar is now 20% faster with the default value

scripts/sfx.py

@@ -234,8 +234,9 @@ def u8(gen):
 
 
 def yieldfile(fn):
-    with open(fn, "rb") as f:
-        for block in iter(lambda: f.read(64 * 1024), b""):
+    s = 64 * 1024
+    with open(fn, "rb", s * 4) as f:
+        for block in iter(lambda: f.read(s), b""):
             yield block