maint: use stat-size module from gnulib

maint: use stat-size module from gnulib

* gnulib: Update to latest.
* src/system.h: Definitions of ST_* macros have moved into the
gnulib module stat-size (specifically, the header file
stat-size.h), so remove them from here.
* src/truncate.c: Include stat-size.h.
* src/stat.c: Likewise.
* src/shred.c: Likewise.
* src/ls.c: Likewise.
* src/du.c: Likewise.
* src/ioblksize.h: New file.  Move definition of io_blksize out of
system.h so that system.h does not have to include stat-size.h.
* src/cat.c: Include ioblksize.h.
* src/split.c: Likewise.
* src/copy.c: Include both stat-size.h and ioblksize.h.
* src/Makefile.am (noinst_HEADERS): Add ioblksize.h.