new module: xprintf

[Jim Meyering]

I've just added a new module:

    http://git.sv.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=78da34d40e6a2

It was prompted by this thread:

    http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/11611/focus=11620