Remove execution domain support

All users of exec_domain are gone, now we can get rid
of that abandoned feature.
To not break existing userspace we keep a dummy
/proc/execdomains file which will always contain
"0-0     Linux                   [kernel]".

Signed-off-by: Richard Weinberger <richard@nod.at>

diff --cc kernel/exec_domain.c
Simple merge

diff --cc kernel/exit.c
Simple merge

diff --cc kernel/fork.c
Simple merge