char * vs. unsigned char * warnings. The GNU coding standards used to
say that these mismatches are okay and better than a bunch of casts.
Obviously this has changed now.
* Makefile.am (libcommon_a_SOURCES): Add xasprintf.c.
* miscellaneous.c (xasprintf): Moved to ...
* xasprintf (xasprintf): ... here. New file.
This allows to use xasprintf without sucking in gpg-error.