Automate illumos sync pull requests

[prakashsurya]

Currently, the openzfs repository is kept in sync with the illumos repository by using git-merge to merge changes from illumos into openzfs.

The process for this is roughly the following series of steps:

1. fetch latest illumos and openzfs changes
2. use git-merge to pull illumos changes into openzfs's "master" branch
3. open a PR of the result of the git-merge
4. wait for testing of the PR to complete/pass
5. git-commit the merged code and close the PR

This would likely be straight forward to automate, and would save us the hassle of doing so manually (as we currently do).