Add iflag=fullblock to dd

rust-coreutils’ dd implementation tends to perform partial writes, which
leads to partial reads and data loss as a result. Strictly speaking,
without the fullblock option there is no guarantee that full blocks will
be read. Even with GNU dd, partial reads and writes can occur, although
they hardly ever happen.

makeself-header.sh

@@ -105,10 +105,10 @@ MS_dd()
 {
     blocks=\`expr \$3 / 1024\`
     bytes=\`expr \$3 % 1024\`
-    # Test for ibs, obs and conv feature
-    if dd if=/dev/zero of=/dev/null count=1 ibs=512 obs=512 conv=sync 2> /dev/null; then
+    # Test for ibs, obs, fullblock and conv feature
+    if dd if=/dev/zero of=/dev/null count=1 ibs=512 obs=512 iflag=fullblock conv=sync 2> /dev/null; then
         dd if="\$1" ibs=\$2 skip=1 obs=1024 conv=sync 2> /dev/null | \\
-        { test \$blocks -gt 0 && dd ibs=1024 obs=1024 count=\$blocks ; \\
+        { test \$blocks -gt 0 && dd ibs=1024 iflag=fullblock obs=1024 count=\$blocks ; \\
           test \$bytes  -gt 0 && dd ibs=1 obs=1024 count=\$bytes ; } 2> /dev/null
     else
         dd if="\$1" bs=\$2 skip=1 2> /dev/null