proc: checks system security policy before trying to get personalities

[HorlogeSkynet]

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See #636.

[mihalicyn]

Dear @HorlogeSkynet,

please add Signed-off-by tag to your commit message.

[HorlogeSkynet]

LGTM, except a few small nits. Thanks!

Thanks for your quick review @mihalicyn. 'just pushed :

• replaced ENOTSUP by EACCES
• added missing \n in (new) error messages
• added an error log in case of read_nointr failure
• changed buf size count
• renamed Yama ptrace scope constants according to Yama internals
• signed-off commit

Bye :wave: