login: actually honour $HOME for chdir()

login: actually honour $HOME for chdir()

When login is almost done it chdir()'s into the user's home directory.
It currently uses pwd->pw_dir, i.e the home dir actually assigned to the
user.

OTOH it explicitly allows overriding HOME (see comment in
init_environ()), and explicitly imports PAM's env vars (which quite
possibly contain $HOME) into its env block.

This patch makes sure that HOME is *properly* overridable via the env
var and via PAM: it actually honours it for the chdir().

(Background: I am working on a PAM module that under some conditions
would like to redirect the home dir to some other dir, and while I can
nicely set $HOME from the PAM module, it has not as much as effect as I
wish, because whie the rest of userspace respects it login so far didn't
and leaves the user in the wrong dir).