Added mremap tracing and fixed a bug in MemoryLimitListener

[mikimasn]

Added mremap syscall tracing in the MemoryLimitListener to catch when the memory limit is exceeded and report a "memory limit exceeded" error instead of returning another possibly unrelated error (segfault, non-zero error code).

I also fixed a bug that caused the error ''memory limit exceeded'' to be displayed when there was no memory limit.
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The error was caused by allocating more than MEMORY_LIMIT_MARGIN/2 bytes of memory in a single mmap call, which resulted in post-allocation memory usage being compared to the memory limit(which was zero).

[Wolf480pl]

Thanks for finding the issue and sending the pull request.

The changes look mostly correct from functionality point of view, most of my remarks are about the coding style. (Which is still important, because it makes it easier to see what's going on, and sio2jail is quite complex.)

The only behavior-affecting concern I have is about the second commit, and this is because there aren't many people with time for maintaining sio2jail so I want to be extra cautious that we don't break anything.

Once you fix that and the stylistic issues, and after we've tested the changes (either I or someone else from the team will try to do it later this week), we'll be able to merge it.

[Wolf480pl]

The code looks good, thanks for making the fixes.

Also, I did some testing, looks like it passes the existing testsuit and I also checked by hand that indeed the two issues with memory limit existed before and are fixed by these changes.

Looks good to go, I'll merge this tomorrow.