Granted, this style is definitely not common, but for the short
utility-functions of this program it's just the right choice. This
provides great flexibility, such that in the long run, it will be
possible to also share code between the OS-implementations.

This also keeps the state-keeping at a minimum and makes it clearer
which functions are implemented on which OS without having to jiggle
around with too many files in the process.

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.

ip.c already works on OpenBSD, it's just missing some headers:

Thank you for your contribution.

Make use of strerror(errno) and format all errors equally:
function ['parameters']: error message

Replace warn() and warnx() with fprintf() and add <stdio.h> where
necessary.

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

Replace #ifdef with #if defined() and #elif with #elif defined() as it
should only test if it is defined or not.

In OpenBSD uptime gets fetched using sysctl now.

In OpenBSD battery percentage gets fetched using apm now.

Print unknown_str in case of error

func can return NULL in case of error, but no checking is done when
printing, and unknown_str wasn't being used anywhere.

When an invalid interface name is passed to wifi_essid() it does not
close opened sockets.