make semantics of custom strndup() comply with glibc

From strdup(3):
| If s is longer than n, only n characters are copied, and a
| terminating null byte ('\0') is added.

So allocate at most n+1 bytes and make sure the last one is zero, as
strncpy() doesn't add it itself.

So in fact to allow a maximum space for string dup of 23, strndup() has
to be called like this:
| dup = strndup(src, 23 - 1);

FIXME: Find the critical points in code this change touches and make
sure the invocation there is correct.

src/common.c

@@ -35,9 +35,10 @@
 // use our own strndup() if it's not available
 char *strndup(const char *s, size_t n)
 {
-	if (strlen(s) + 1 > n) {
-		char *ret = malloc(n);
+	if (strlen(s) > n) {
+		char *ret = malloc(n + 1);
 		strncpy(ret, s, n);
+		ret[n] = 0;
 		return ret;
 	} else {
 		return strdup(s);