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

tests/util.py

@@ -147,6 +147,7 @@ class Cfg(Namespace):
             dbd="wal",
             fk_salt="a" * 16,
             idp_gsep=re.compile("[|:;+,]"),
+            iobuf=256 * 1024,
             lang="eng",
             log_badpwd=1,
             logout=573,