Based on functionality in dstat by Joerg Jung.

On OpenBSD the backlight percentage cannot be retrieved in a simple way.
The only two solutions we are aware of for now are:

 - reading from /dev/ttyC0: which isn't possible without changing
 permissions or running slstatus as root
 - linking against xcb-xrandr: which is bloat and does not work in every
 case appearently

This reverts commit 37724ac2c3 for now.

At the moment linux only, but will add support for OpenBSD as well.

Additionally unify the format of battery_state and uptime

OpenBSD's entropy design is superior to Linux.

The third value from load_avg (idle) gives us almost the same
information as cpu_iowait. Plus OpenBSD does not offer an iowait value
as Linux and thus the corresponding function would not be portable.

In OpenBSD the CPU usage in percent is now computed using KERN_CPTIME
sysctl.

- Battery power cannot be easily gatherable via apm(4)
- IMHO it does not represent essential information

this reverts the commits from 92ab9ef52e up to
d42870d6ca.

After heavy consideration, the component split has more disadvantages
than advantages, especially given there will be utility-functions
sharing quite a lot of code that would then need to be duplicated, as it
does not fit into the util.c due to its speciality.

One big advantage of the component-wise build is readability, and
without doubt, this was achieved here. This point will be addressed
with a different approach that will be visible in the upcoming commits.

One big disadvantage of the component build is the fact that it
introduces state to the build process which is not necessary. Before its
introduction, the only influencing factors where the system-defines
__linux__ and __OpenBSD__. With the components, we are also relying on
the output of uname(1).
Additionally, if the os.mk is not present, make gives the output

     $ make
     Makefile:5: os.mk: No such file or directory
     make: *** No rule to make target 'os.mk'.  Stop.

This could easily be fixed by providing some sort of meta-rule for this
file, however, it indicates the problem we have here, and this entire
statefulness will heavily complicate packaging of this tool and makes
the build process much more complex than it actually has to be.

In OpenBSD CPU frequency gets fetched using sysctl now.

In OpenBSD temperature gets fetched using sysctl now.

ALSA is not a dependency anymore as we use /dev/mixer. The corresponding
notice about PulseAudio is thus not necessary too.

In OpenBSD uptime gets fetched using sysctl now.

In OpenBSD battery percentage gets fetched using apm now.

- Renamed "ip" function to "ipv4"
- Added "ipv6" function
- Adjusted README and config.def.h accordingly