misc1: Split out line_consumption

- Move all functions regarding physical line consumption into line_consumption.c
- Convert and add comments in doxygen style
- Create header line_consumption.h for inclusion by users of these functions
- Include this header in all files using plines_* functions
line_consumption: Add to clint-files.txt and fix errors

Add the files line_consumption.[ch] to clint-files.txt and fix all errors
detected by clint.
line_consumption: Add -Wconversion for line_consumption

- Add line_consumption.c to the files that get compiled with -Wconversion
- fix warnings resulting from -Wconversion
line_consumption: Fix coding style and types

- Format code acording to style guide
- Change return types and types used in functions
line_comsption: Rename to screenline.[ch]

Problem:
  * The file names for line_consumption.[ch] were not in line with
    other file names that don't contain underscores.
  * The file names were not descriptive enough for their function.

Solution:
  * Rename the files to screenline.[ch] and change all occurences.