build: update to latest gnulib

build: update to latest gnulib

Included in this are gnulib changes 3ea43e02 2768ceb7
which make the device IDs from /proc/self/mountinfo
available to df.  This can be leveraged by a subsequent
change to df to present a more accurate list of file systems.

* bootstrap: Merge from gnulib.
* src/ls.c (dev_ino_pop): s/obstack_blank/obstack_blank_fast/
as this API/ABI has changed, giving memory exhausted errors
if negative (large positive) numbers are passed to obstack_blank().
* tests/df/skip-duplicates.sh: Adjust as the new gnulib code
requires a non NULL mnt_opts even when mnt_type is not "none".