Co-authored-by: drkhsh <me@drkhsh.at>
Signed-off-by: drkhsh <me@drkhsh.at>

Generically reads an arbitrary file natively.

Saves a few layers of execution in comparison to using `run_command`
with an argument like `cat ./file`.

Just use the format strings dude. :)

function prototype with unspecified argument is obsolete since c99.

additionally some of these function which don't take any argument were
being called with a `const char *` arg, which is UB.

fix both these issues by declararing ALL the components to accept a
`const char *`, and name the arg "unused" if it's meant to be ignored.

according to both the function definition and the comment in
config.def.h, keyboard_indicators() should take a format string.

Function declarations should include variable names.
For functions with different parameters on different platfroms they are
left out.

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 37724ac2c3f496f4736223d0d8e5d8fecb933590 for now.