Additionally unify the format of battery_state and uptime

when an AC is connected apm_info shows a non-valid value for remaining
minutes. it was decided that in that case the function should return an
empty string.

Tabs for indentation, spaces for alignment

Implementation of a battery_remaining function which returns the
remaining battery time in HH:MM format. Linux function still needs
implementation.
Move common code to load_apm_power_info

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

int for normal indexes
size_t for usage with the LEN macro

- Use block for single statement ifs
- Keep lines to reasonable length (current debate as to reasonable)
- When functions return -1 for error test against 0 not -1
- Do not indent cases another level
- Do not test against NULL and 0 explicitly
- Use tabs for indentation, use spaces for alignment

It does not respect charging thresholds and it is not trivial to
implement the indicator in OpenBSD.

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

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.

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.

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

In OpenBSD battery percentage gets fetched using apm now.

This brings us a lot more tidiness.

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