Added POSIX-compliant updating of mupdf

[Aleppi]

This is meant to be a POSIX-compliant version of #59 since it hadn't been updated in a month.

It finds the PIDs of all mupdf processes and sends SIGHUP to them. No mupdf instance is closed, including other ones running in the background, however I am pretty sure they still recieve the SIGHUP sent.

I haven't found a way to get the PID from the mupdf process that gets started, $! doesn't seem to work to get the PID of the started process.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 30 days if no further activity occurs, but feel free to re-open a closed issue if needed.
Thanks for contributing to vim-llp!

[xuhdev]

I personally feel like this changes normally make the code difficult to maintain. I would rather wrap mupdf in a script and let Vim call that script. Would you mind adding this to Wiki instead of changing the code? https://github.com/xuhdev/vim-latex-live-preview/wiki/Known-Working-PDF-Viewers