Pull printf-related code from gnulib, rather than using forked copy.

Pull printf-related code from gnulib, rather than using forked copy.
* bootstrap.conf (gnulib_modules): Don't avoid size_max and xsize
modules.  While I dislike xsize-style overflow avoidance, maintaining
a forked version of e.g., vasnprintf.c was too much work.
* lib/printf-parse.c, lib/vasnprintf.c, lib/unicodeio.c: Remove local
copies, so we now get these files from gnulib.