CVE-2012-3524: Hardening for being run in a setuid environment

Some programs attempt to use libglib (or even libgio) when setuid.
For a long time, GTK+ simply aborted if launched in this
configuration, but we never had a real policy for GLib.

I'm not sure whether we should advertise such support.  However, given
that there are real-world programs that do this currently, we can make
them safer with not too much effort.

Better to fix a problem caused by an interaction between two
components in *both* places if possible.

This patch adds a private function g_check_setuid() which is used to
first ensure we don't run an external dbus-launch binary if
DBUS_SESSION_BUS_ADDRESS isn't set.

Second, we also ensure the local VFS is used in this case.  The
gdaemonvfs extension point will end up talking to the session bus
which is typically undesirable in a setuid context.

Implementing g_check_setuid() is interesting - whether or not we're
running in a privilege-escalated path is operating system specific.
Note that GTK+'s code to check euid versus uid worked historically on
Unix, more modern systems have filesystem capabilities and SELinux
domain transitions, neither of which are captured by the uid
comparison.

On Linux/glibc, the way this works is that the kernel sets an
AT_SECURE flag in the ELF auxiliary vector, and glibc looks for it on
startup.  If found, then glibc sets a public-but-undocumented
__libc_enable_secure variable which we can use.  Unfortunately, while
it *previously* worked to check this variable, a combination of newer
binutils and RPM break it:
http://www.openwall.com/lists/owl-dev/2012/08/14/1



So for now on Linux/glibc, we fall back to the historical Unix version
until we get glibc fixed.

On some BSD variants, there is a issetugid() function.  On other Unix
variants, we fall back to what GTK+ has been doing.

Reported-By: Sebastian Krahmer <krahmer@suse.de>
Signed-off-by: Colin Walters <walters@verbum.org>