Fix compile error on Linux and update Makefile

[kinggizzardandtheinstallationwizard]

There is a fatal error in local.c that occurs in Linux but not Darwin/MacOS (unsure if other platforms are affected) if unistd.h is not included.

I also went ahead and moved all of the parts of the Makefile that vary per end-user to a new config.mk file. Additionally I bumped terminfo to the top of the list over termcap because that has been the more common option for a very long time now.

[justwheel]

@jamesjer opened a downstream Fedora Linux RPM pull request to the package spec for zork in Fedora. I am hoping that we can work together to upstream those changes, including the use of terminfo instead of termcap, as part of a series of subsequent PRs to keep zork compiling for Linux users for generations to come.

I have not taken a close look to see how this PR differs from @jamesjer's patches yet. He also did some modernization efforts to C23.

[kinggizzardandtheinstallationwizard]

I skimmed over it a little. The modernization work looks wonderful, a lot of much-needed changes. The changes in my PR do not touch much code at all, I just added a single include and rearranged the Makefile a little.

[justwheel]

This PR may be superseded by #31 and #32.

[justwheel]

All other PRs have merged, and this now looks in conflict with the current changes in main. I think we have effectively resolved the compile errors and improved the Makefile instructions. We can always use the CI pipeline to test any future changes.

Thanks for your interest in contributing to Zork, @kinggizzardandtheinstallationwizard! 🙏🏻 Let us know if you have any new issues with the recent changes.