obstacks: use obstack_blank_fast where possible.

In preparation for gnulib API change to obstacks, where
obstack_blank cannot shrink memory use any more.
* m4/macro.c (trace_flush): `start - len` can be negative, so
call obstack_blank_fast, which accepts negative arguments.
* modules/m4.c (m4_make_temp): Likewise... -1 is always negative!

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
gnulib: sync with upstream.

Notably this moves to the new 64bit obstack implementation.
* gnulib: Sync with upstream, for maint.mk bug fixes et al.
* build-aux/m4/gnulib-cache.m4, po/Makevars: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>