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.

libdbus abort()s when a string argument is not valid UTF-8. Since the
arguments sometimes come from untrusted sources, it's better to check
the string validity explicitly.

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.

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

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

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

If leak_check_setup is not called then global_talloc_context
was not initialized and check_leaks_pop(global_talloc_context) will fail.

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

If we are already requested used then we needn't to call
setreeuid(), setresgid(). But we forgot to relase local
struct sss_creds *ssc, which is used for returnig saved credentials.

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

When ifp_users_user_get_groups is called, for example via GetAll and
the list of groups contains a non-POSIX group, we skip an array member,
resulting in random memory being passed to the caller.

Resolves:
    https://fedorahosted.org/sssd/ticket/2863

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