Add compat strndupa() implementation from libnostd

This patch adds compat support, to *BSD and other UNIX, for the GLIBC
strndupa() function.  The implementation is taken from libnostd written
by William Ahern.

http://www.25thandclement.com/~william/projects/libnostd.html

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

strlite.h

@@ -26,6 +26,7 @@
 
 #include <string.h>
 #include "strdupa.h"
+#include "strndupa.h"
 
 #ifndef strlcpy
 size_t  strlcpy    (char *dst, const char *src, size_t siz);