secure exec layering violation

The way apparmor sets up and tracks the need for secureexec between security_bprm_set_creds and security_bprm_secureexec is wrong.

apparmor sets an unused bit in the bprm->unsafe flag set. This has a few problems
1. Its unreserved so future code may use that bit
2. bprm code code clear, set the entire unsafe flag set instead of doing bit setting
3. lsms can now be stacked so multiple lsms may be looking at the flag set and having unknown bits set may cause a problem

None of these currently occur but they could and the problem would not be immediately apparent.

Since it is apparmor specific, the flag should be tracked in an apparmor specific way. Likely off of the apparmor part of the cred in the bprm.