restorer: allocate a stack guard page

[avagin]

An accidental overflow could corrupt the area located below a stack, so let's map an extra page there without read-write permissions.

[0x7f454c46]

I guess, there are still two issues I see:

1. bootstrap_len is a continuous block so we might fail to find this large amount for restorer - may be worth to split it on multiple vmas (though, it may be not a problem on 64-bit systems).
2. Now the redzone is one page, but in the original failure, it was an array that has a couple of kilobytes size, which means 4Kb maybe not enough for structures used on restorer's stack.

Anyway, the patch seems good to me and is a nice start that may catch new issues. With a minor nit, you can add my Reviewed-by: Dmitry Safonov <[email protected]> or which way is now preferred :) (I don't have a button that would mark PR as reviewed)

[0x7f454c46]

@avagin ping? :)

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