Optimize f_sprintf() for bash

bash lacks the ksh93 optimization that makes sub-shells fast if they do
not alter io. bash 3.1-alpha1 introduced printf -v var_to_set which is not
as fast but is still significantly faster than var_to_set=$( printf ) when
using any version of bash. If we find our interpreter to somehow be bash
by invocation or inclusion, use the feature that provides fastest results.