size_t may not be enough, use 64 bit integers

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.

It is not necessary to copy memory or anything. Just keep a pointer to
the active prefix-array and assign the length of the arrays to a
variable.

Make the code more readable by using a switch, be more strict when an
invalid base is passed to it and fix a small oversight in the bottom of
the code where the base 1024 was forgotten to generalized.

Now only one function, fmt_human, takes an additional argument "base".

This is a first step to decouple formatting from information because of
two reasons:

 1. The components should only gather and return the values by design
 2. Fine grained user control should be a focus

Scaling will be implemented in a different way in a later commit.

These functions take the raw number and a unit and automatically
print it out "scaled down" to a proper SI-prefix, for powers of 2
and 10 respectively.

Apply them to the 2-power cases and keep the 10-power for a later
commit.

Within the components, snprintf() was unchecked and had inefficient
calls in some places.
We implement esnprintf() that does all the dirty laundry for us
and use it exclusively now.

Given slstatus is a tool that runs in the background, most likely run
from .xinitrc, it's important to prepend the name of the tool to error
messages so it becomes clear where the error is coming from.

To make this much more consistent, this commit adds warn() and die()
utility functions consistent with other suckless projects and adapts all
calls to fprintf(stderr, *) to the warn() and die() functions, greatly
increasing the readability of the code.

Use variables named unused instead which is simpler

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