Based on functionality in dstat by Joerg Jung.

Remove unnecessary variable

Thanks to the FRIGN

Remove unnecessary cast to size_t

Updated for style.

- Remove <errno.h> because related functions are in util.c now
 - Remove corresponding <string.h> if not used otherwise

This drops a lot of code and even though we are using a common function,
the specification of only the needed things keeps it very efficient and
even more efficient in memory and time than the previous solution.

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.

- Get rid of camel-casing
 - Don't use all-caps for variable names
 - use LEN()-macro
 - use strncmp() rather than strstr() for prefix-checking
 - clean up the tokenizer-loop and don't use copies
 - make the loop more readable by separating different breaking
   conditions
 - stricter error-checking and cleanup
 - store the layout directly with bprintf rather than having
   a separate buffer

Adding a new keymap component that will
indicate the current keyboard layout (language)
and variant if any was set. I use the
standard X11 XKB APIs to retrieve and parse
the xkb_symbols set with setxkbmap.

At the moment linux only, but will add support for OpenBSD as well.

Thanks to Hiltjo

Add a return value so we are able to check any errors and actually fail
when there has been a critical error instead of carrying on uselessly.

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

In a later commit something like that will be enabled for every
component