Add systemd (libsystemd and libudev outputs only!)

[ryanvolz]

This adds a proper conda package for libudev (and libsystemd just because), which is desirable over the current state of using a CDT because of conda-forge/cdt-builds#48 and the general headaches that come with depending on CDTs (e.g. conda-forge/libusb-feedstock#15). I've tested this locally with a rebuild of libusb, and it appears to work.

This builds with no patches for the CentOS 7 base environment, and I think it's probably sufficient to leave it at that and continue to use the libudev CDT for cos6. Judging from the previous packaging attempt, it should be possible to patch this up to work with the older glibc and kernel versions used with CentOS 6 should that be desired.

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[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipes/systemd) and found it was in an excellent condition.

[ryanvolz]

@conda-forge/staged-recipes Ready for review. This one could probably use some extra careful eyes.