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.

Implement basic percentage calculation for wifi_perc. With this
commit, the function returns correct percentage, since the max
of what /proc/net/wireless returns is 70.

This brings us a lot more tidiness.

The HDR variable is used for general purpose headers, which are not
given in this project. arg.h and config.h are only needed for
slstatus.c, not every component $(COM:=.c).

This also fixes a problem introduced in
e04a3853643df1c98a451969d515d8850f138bcc, where config.h is pulled into
the tarball.

For multiple reasons the program is now split:
- Make future porting to OpenBSD easier
- Assign header includes to individiual functions
- Make future program extensions easier
- Recompile only changed parts