Proper formatting of the license text

The license file was totally fucked up and non-readable, without any
paragraph distinction nor alignment.

I retrieved the GNU GPLv2 license from
<https://gnu.org/licenses/gpl-2.0.txt> and updated the license file.
Merge branch 'license-format' of https://github.com/valeriangalliat/spawnkill into valeriangalliat-license-format