Fix non-functional chdir()

I don't know how chdir() ever worked - apparently it didn't!

It took an argument "pathname", and then declared a local "path" and used
that, not pathname, as the path :-) Obviously, a call to task_conv, which
converts a relative "pathname" to an absolute "path", was missing...

chdir() is still a mess and incompatible in the error cases with Linux's
chdir(). I'll fix that, and add a test, in a follow-up patch.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>