replaced stdndup() with the inlined equivalent

don't kill the OS incompatibility messenger.

1)
http://stackoverflow.com/questions/6062822/whats-wrong-with-strndup
stdndup() is POSIX 2008, but apparently not available on OSX and Windows
it could be made potentially application global (e.g. a local "stdndup.h")

2)
free() memory at pointer "current_dir", once we are done.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>