Add xasprintf() and xvasprintf()

This adds wrappers for xasprintf() to replace the ridiculous hand-rolling of
malloc with strcpy/strncpy, etc., etc.
Audit:  libs/Bindings.[ch]

* Some reindentation of the code.
* Forward-declare static functions.
* Replace instance of alloca()/strcpy with xasprintf()
Audit:  libs/ClientMsg.[ch]

* Reindent to comply with style.