build: uname: avoid shadowing warning

* src/uname.c (main) [__APPLE__]: Rename inner "s" to "cs",
to avoid gcc shadowing warning.
stdbuf: support OS X

* src/stdbuf.c (set_LD_PRELOAD): Adjust to use Mac OS X
specific environment variables on __APPLE__ platforms.
Fixes http://bugs.gnu.org/17590
df: use the last device name provided by the system

The device name reported for a particular mount entry
may no longer be valid if the mount point was subsequently
mounted on a different device.  Therefore honor the order
of the mount list returned by the system and use the last
reported device name.

* src/df.c (filter_mount_list): When discarding the current
mount entry, ensure that a new device name is not also discarded.
* tests/df/skip-duplicates.sh: Add a test case.  Also fix
a false failure in the edge case of a system with only a
single file system.
* NEWS: Mention the fix.
stat: avoid redundant stat() calls

* src/stat.c (find_bind_mount): NAME is invariant in the loop,
so only stat(NAME) outside the loop.
cut: restore special case handling of -f with -d$'\n'

commits v8.20-98-g51ce0bf and v8.20-99-gd302aed changed cut(1)
to process each line independently and thus promptly output
each line without buffering.  As part of those changes we removed
the special handling of --delimiter=$'\n' --fields=... which
could be used to select arbitrary (ranges of) lines, so as to
simplify and optimize the implementation while also matching the
behavior of different cut(1) implementations.

However that GNU behavior was in place for a long time, and
could be useful in certain cases like making a separated list like
`seq 10 | cut -f1- -d$'\n' --output-delimiter=,` although other tools
like head(1) and paste(1) are more suited to this operation.
This patch reinstates that functionality but restricts the
"line behind" buffering behavior to only the -d$'\n' case.

We also fix the following related edge case to be more consistent:

  before> printf "\n" | cut -s -d$'\n' -f1- | wc -l
  2
  before> printf "\n" | cut    -d$'\n' -f1- | wc -l
  1
  after > printf "\n" | cut -s -d$'\n' -f1- | wc -l
  1
  after > printf "\n" | cut    -d$'\n' -f1- | wc -l
  1

* src/cut.c (cut_fields): Adjust as discussed above.
* tests/misc/cut.pl: Likewise.
* NEWS: Mention the change in behavior both for v8.21
and this effective revert.
* cfg.mk (old_NEWS_hash): Adjust for originally omitted v8.21 entry.
* src/paste.c: s/delimeter/delimiter/ comment typo fix.
build: be more specific about .git repo before enabling warnings

* configure.ac: When looking for a .git checkout, exclude repos that
contain a .tarball-version file as these are probably releases
that are imported into git for patch management.
maint: avoid compiler warnings with some assert() implementations

* src/df.c (get_dev): asssert() on Solaris 10 is not marked as
__noreturn__ and thus the compiler may think V is uninitialized
later on in the function.
* THANKS.in: Remove the now committer.