obstack: merge with glibc changes

* lib/obstack.c, lib/obstack.h: Merge from glibc.
This is mostly indenting and commentary changes.
Instances of 'register' have been removed.
obstack: Remove ancient NeXTSTEP gcc support conditional

This change will ease merging with glibc.  The "#if ... __NEXT__"
causes a warning with -Wundef which glibc now enables by default.
Problem reported by Will Newton in
<http://lists.gnu.org/archive/html/bug-gnulib/2014-03/msg00032.html>.
glibc <sys/cdefs.h> now uses __extension__ for GCC 2.8 or later,
so go with that.
* lib/obstack.h (__extension__):
autoupdate
autoupdate