maint: simplify memory alignment

maint: simplify memory alignment

Use the new Gnulib modules alignalloc and xalignalloc
to simplify some memory allocation.
Also, fix some unlikely integer overflow problems.
* bootstrap.conf (gnulib_modules): Add alignalloc, xalignalloc.
* src/cat.c, src/copy.c, src/dd.c, src/shred.c, src/split.c:
Include alignalloc.h.
* src/cat.c (main):
* src/copy.c (copy_reg):
* src/dd.c (alloc_ibuf, alloc_obuf):
* src/shred.c (dopass):
* src/split.c (main):
Use alignalloc/xalignalloc/alignfree instead of doing page
alignment by hand.
* src/cat.c (main):
Check for integer overflow in page size calculations.
* src/dd.c (INPUT_BLOCK_SLOP, OUTPUT_BLOCK_SLOP, MAX_BLOCKSIZE):
(real_ibuf, real_obuf) [lint]:
Remove; no longer needed.
(cleanup) [lint]:
(scanargs): Simplify.
* src/ioblksize.h (io_blksize): Do not allow blocksizes largest
than the largest power of two that fits in idx_t and size_t.
* src/shred.c (PAGE_ALIGN_SLOP, PATTERNBUF_SIZE): Remove.