remove non-portable usage of strndupa

usage of strndupa is neither C99 nor POSIX,
it's a glibc invention, and a dangerous one since
alloca() is used behind the scenes which does not
give any guarantuees that it won't overflow the 
stack.

--- libkmod/libkmod-util.c	2013-08-26 16:03:22.239000003 +0000
+++ libkmod/libkmod-util.c	2013-08-26 16:07:26.684000003 +0000
@@ -28,6 +28,7 @@
 #include <unistd.h>
 #include <errno.h>
 #include <string.h>
+#include <limits.h>
 #include <ctype.h>
 
 #include "libkmod.h"
@@ -323,8 +324,11 @@
 int mkdir_p(const char *path, int len, mode_t mode)
 {
 	char *start, *end;
-
-	start = strndupa(path, len);
+	char buf[PATH_MAX+1];
+	snprintf(buf, sizeof buf, "%s", path);
+	assert(len < sizeof(buf));
+	buf[len] = 0;
+	start = buf;
 	end = start + len;
 
 	/*