Replace use of (v)snprintf with (v)asprintf. This has several advantages:

Replace use of (v)snprintf with (v)asprintf. This has several advantages:

1. (Mostly) easier to program with as the allocation is done for you.
   (Sometimes it's a little more complicated to get the deallocation
   right). The amount of code removed as a result by this commit is
   greater than the amount introduced (excluding the (v)asprintf
   implementation).

2. Safer: no buffer overflows.

3. Works on all platforms: BSD-licensed mature xnprintf code used
   (I've been using this for some years in Zile with no problems). No
   more overflows due to the previous bogus snprintf/vsnprintf
   fallbacks.

Also add and use a MAX() macro in the same vein as MIN().