Restore sprintbuf(), add macro for string literals

Hawciz pointed out that the previous commit modifies the public
interface of printbuf. Per his suggestion, sprintbuf() was restored
and a new pair of macros was added that wraps printbuf_memappend().

Using a wrapper macro instead of modifying sprintbuf() also reduces
function call overhead, bringing total performance gains to
approximately 400%.

printbuf.c

@@ -85,7 +85,6 @@ int printbuf_memappend(struct printbuf *p, const char *buf, int size)
     if (printbuf_extend(p, p->bpos + size + 1) < 0)
       return -1;
   }
-
   memcpy(p->buf + p->bpos, buf, size);
   p->bpos += size;
   p->buf[p->bpos]= '\0';
@@ -112,6 +111,34 @@ int printbuf_memset(struct printbuf *pb, int offset, int charvalue, int len)
 	return 0;
 }
 
+int sprintbuf(struct printbuf *p, const char *msg, ...)
+{
+  va_list ap;
+  char *t;
+  int size;
+  char buf[128];
+
+  /* user stack buffer first */
+  va_start(ap, msg);
+  size = vsnprintf(buf, 128, msg, ap);
+  va_end(ap);
+  /* if string is greater than stack buffer, then use dynamic string
+     with vasprintf.  Note: some implementation of vsnprintf return -1
+     if output is truncated whereas some return the number of bytes that
+     would have been written - this code handles both cases. */
+  if(size == -1 || size > 127) {
+    va_start(ap, msg);
+    if((size = vasprintf(&t, msg, ap)) < 0) { va_end(ap); return -1; }
+    va_end(ap);
+    printbuf_memappend(p, t, size);
+    free(t);
+    return size;
+  } else {
+    printbuf_memappend(p, buf, size);
+    return size;
+  }
+}
+
 void printbuf_reset(struct printbuf *p)
 {
   p->buf[0] = '\0';
@@ -125,8 +152,3 @@ void printbuf_free(struct printbuf *p)
     free(p);
   }
 }
-
-inline int sprintbuf(struct printbuf *p, const char *buf)
-{
-  return printbuf_memappend(p, buf, strlen(buf));
-}