helpers.h: add xasprintf function
helpers.h: add xasprintf function

This is our own version of asprintf(). This original code was written by
Mike Frysinger, and I was able to modify it to use our memory helper
functions.

We need a version of this code because it is not available on glibc at
least without defining _GNU_SOURCE, and I would rather not do that.

This is the first step in improving string handling in OpenRC for #207.

dwfreed/openrc,williamh/openrc,dwfreed/openrc,williamh/openrc,OpenRC/openrc,williamh/openrc,OpenRC/openrc,OpenRC/openrc,dwfreed/openrc