kern/21463: [linux] Linux compatability mode should not allow setuid programs

[Marcin Cieslak]

We implement AT_UID and AT_GID process auxillary vector
(procstat -x) elements so at least userland library
(such as glibc) has a possibility to enter "secure mode"
on startup.

Not sure if there is anything we can do more (except
for disabling the feature altogether) to tell userland
to be careful.

//Marcin