hardening: Use __secure_getenv() in *addition* to _dbus_check_setuid()

hardening: Use __secure_getenv() in *addition* to _dbus_check_setuid()

This is a further security measure for the case of Linux/glibc
when we're linked into a binary that's using filesystem capabilities
or SELinux domain transitions (i.e. not plain old setuid).

In this case, _dbus_getenv () will return NULL because it will
use __secure_getenv(), which handles those via AT_SECURE.

https://bugs.freedesktop.org/show_bug.cgi?id=52202