Fixes:
    /sssd/src/tools/sss_override.c: In function ‘override_user_import’:
    /sssd/src/tools/sss_override.c:1471: warning: declaration of ‘exit’ shadows a global declaration
    /usr/include/stdlib.h:544: warning: shadowed declaration is here
    /sssd/src/tools/sss_override.c: In function ‘override_group_import’:
    /sssd/src/tools/sss_override.c:1737: warning: declaration of ‘exit’ shadows a global declaration
    /usr/include/stdlib.h:544: warning: shadowed declaration is here

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.

We could use SSS_DFL_UMASK instead of DFL_UMASK.

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

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

There is no need to have executable unique_file.

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

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