Increase buffer size for formatted numeric quantities, to avoid truncation.

This can happen if the user forces a particular output format that leads
to many digits (6 or more) being printed.  The new buffer size is probably
larger than it needs to be, but better safe than sorry.

Fixes Issue 142.

src/units.h

@@ -8,7 +8,7 @@
  */
 
 enum {
-    UNIT_LEN = 11
+    UNIT_LEN = 32
 };
 
 double unit_atof( const char *s );