* src/dd.c: Detect some very unlikely buffer overflows.

* src/dd.c: Detect some very unlikely buffer overflows.
(INPUT_BLOCK_SLOP, OUTPUT_BLOCK_SLOP): New macros.
(MAX_BLOCKSIZE): Now accepts an arg.  All uses changed.
(page_size): New var.
(scanargs, skip, main): Use more-straightforward way to detect overflow.
(dd_copy): Use page_size rather than invoking getpagesize.
Use INPUT_BLOCK_SLOP, OUTPUT_BLOCK_SLOP.
(main): Set page_size.
Avoid a call to stat in the usual case where ftruncate succeeds.