The XDG Base Directory specification [1] demands that there should be a
"single base directory relative to which user-specific non-essential
(cached) data should be written". Light currently makes use of such a
cache directory, but hardcodes the default "$HOME/.cache".

In order to enable users to specify a custom cache directory, respect
this convention and use the "$XDG_CACHE_HOME" environment variable if it
is set. If it is not set, fall back to "$HOME/.cache".

[1] https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

When light is installed in the non-SUID root mode it cannot write to
/etc/light anymore, so we set up a ~/.cache/light instead.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

- Use version from configure script
- Simplify usage text for accessibility, only one command at a time
- Simplify terminology, we have commands and options to commands

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

Without this patch -O and -I are not supported anymore and lead to an
error.

- Use NAME_MAX and PATH_MAX instead of hardcoded values
- Allow paths to be longer than 256 chars
- Check pointers everywhere
- Use strncpy/snprintf instead of strcpy/sprintf
- Validate controllers' name (-s flag + a very long name -> bad things happening)
- Get rid of globals for dir iteration

- Rewrite of all clamp operations
- Get rid of unreachable code (the 'else' in the last chunk)

Now we can save, restore, get [max] brightness etc. for both screen
and keyboard controllers

This function was:
- Reading values from the controller
- Checking write permission even when we just want reading values
- Checking the mincap file instead of the actual controller

The last one is also true for other functions using
'light_getBrightness'.
This commit fixes that.