Make all indentation consistent.

Previously, there were many indentation styles in use: Real tabs, 2, 3
or 4 spaces, mixing these within the same file, function or even within
a single line.

This commit changes all bash scripts in the handlers, src and lib dirs
to use consistent indentation: three spaces are now used everywhere.
Other files (e.g. Makefiles) are left untouched.

Additionally, this commit removes all trailing whitespace.