The handling of sbus requests in Data Provider was extremely duplicated,
resulting in copy-paste errors. This patch adds several helpers that
reduce code duplication.

https://fedorahosted.org/sssd/ticket/2861

Messages passed from Data Provider to responder must be valid UTF-8
strings. Because providers might not be completely under our control,
we need to check if the messages we receive are valid UTF-8 and if they
are not, use a fallback.

Resolves:
    https://fedorahosted.org/sssd/ticket/2861

All back end requests were using pam_strerror() to print additional info
about why request failed. Since pam_strerror() returns localized message
and we don't know the locale beforehand, this message failed to be
transferred through D-Bus, resulting in a crash.

We could use more restrictive umask in krb5_child. I found out that
there is directory creation, but it is done by create_ccache_dir()
which has its own umask setup.

Resolves:
https://fedorahosted.org/sssd/ticket/2424

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Set PAM_SYSTEM_ERR as default pam_status to ensure that we always must
set PAM_SUCCESSS explicitly for a successful authentication and will
really return an error in all other cases.

Reviewed-by: Pavel Reichl <preichl@redhat.com>

Reviewed-by: Petr Cech <pcech@redhat.com>

Reviewed-by: Pavel Reichl <preichl@redhat.com>

Update get next domain to be able to
include disbled domains and change the
interface to accept flags instead of
multiple booleans.

Ticket:
https://fedorahosted.org/sssd/ticket/2673

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Resolves:
    https://fedorahosted.org/sssd/ticket/2829

Reviewed-by: Pavel Březina <pbrezina@redhat.com>