Honor pam_unix.so return value

Commit eb138196aa2d3cb860d5eb5ab1d05985df34ad2c changed the return value
of pam_authenticate() for the case when the user enters an incorrect
password.  Prior to that change pam_authenticate() would return
PAM_AUTH_ERR for an incorrect password, while after it would return
PAM_PERM_DENIED.

The root cause is that after that change, nothing in the stack before
the final pam_faillock.so auth entry is setting `impression` in
_pam_dispatch_aux().  If the user has not reached the maximum number of
tries, pam_faillock.so returns PAM_IGNORE [1] and thus
_pam_dispatch_aux() sets `status` to PAM_MUST_FAIL_CODE [2], which is
defined to be PAM_PERM_DENIED [3].  This ends up being the return value
for pam_authenticate().

This commit addresses the problem by changing the `default` control
action for the pam_unix.so auth entry from `ignore` to `bad` (the same
as when its control value was `required`).  Thus when processing the
pam_unix.so entry, _pam_dispatch_aux() will set `impression` to
_PAM_NEGATIVE and `status` to the return value of pam_unix.so,
PAM_AUTH_ERR [4]. _pam_dispatch_aux() will then continue to the final
pam_faillock.so auth entry.  Because `impression` is now _PAM_NEGATIVE,
_pam_dispatch_aux() will not change the value of `status` and the return
value of pam_authenticate() is PAM_AUTH_ERR as desired.

Also ensure that `new_authtok_reqd` is handled correctly when returned
from from pam_unix.so.

[1] https://github.com/linux-pam/linux-pam/blob/d3b73b6cd818f4fd9c923822592eccbe8ecdd121/modules/pam_faillock/pam_faillock.c#L712

[2] https://github.com/linux-pam/linux-pam/blob/d3b73b6cd818f4fd9c923822592eccbe8ecdd121/libpam/pam_dispatch.c#L244

[3] https://github.com/linux-pam/linux-pam/blob/d3b73b6cd818f4fd9c923822592eccbe8ecdd121/libpam/pam_dispatch.c#L17

[4] https://github.com/linux-pam/linux-pam/blob/d3b73b6cd818f4fd9c923822592eccbe8ecdd121/libpam/pam_dispatch.c#L246

Signed-off-by: Daniel Harding <dharding@living180.net>
Closes: https://github.com/gentoo/pambase/pull/10
Signed-off-by: Sam James <sam@gentoo.org>

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