Fix signedness errors as part of -Werror cleanup.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Automatically reformat com32/libutil/sha512crypt.c using Nindent.

Do this for all files except HDT, gPXE and externally maintained
libraries (zlib, tinyjpeg, libpng).

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

Use "-nostdinc -iwithprefix include" to prevent the inclusion of
system header files, but still permitting *compiler* header files.

Add a common header file, xcrypt.h, for the various extended crypt
routines (md5, sha256, sha512).

Add support for sha256crypt and sha512crypt to the library.  Not yet
included in the menu system, need to decide if it's worth 10K or if
the code can be realistically compacted.