illumos gate

I spent quite some time looking at the system call man-pages whilst porting Zig to Illumos, and found that they were somewhat lacking.
As a casual technical writer and mdoc enthusiast, I decided to rewrite them all in mdoc.
You can review the in-progress work at my fork¹, the commit message has more information.

[1]: https://github.com/gh-fork-dump/illumos-gate/tree/syscall-mdoc-reformat