Drop sys/time.h and unistd.h

[wegank]

If I'm not mistaken, unistd.h was never actually used, and we only use gettimeofday from sys/time.h, which can easily be replaced with the modern (and more accurate) timespec_get from the already-imported time.h. So we can drop them both, I think?

[wegank]

The failing test is reproducible on my aarch64-windows machine with the given seed.

$ ./msolve.exe -f input_files/eco10-31.ms -o out.ms --random-seed 1779878689 -P 2 -d 0 -L 0 -l 44 -t 1 -v 2
Initial seed for pseudo-random number generator is 1779878689
...
------------------------------------------------------------------------------------
msolve overall time           1.71 sec (elapsed) /  1.71 sec (cpu)
------------------------------------------------------------------------------------
$ cat out.ms

$

I can look at it in two weeks. Otherwise, since Windows is Tier 3, this should not block the PR.

[wegank]

Oh, actually this is not Windows-specific; I can reproduce on macOS by modifying the rand function. I'll open an issue.