This removes script-wide shellcheck excludes to avoid unintentional
errors and moves the excludes to the scripts where they are necessary.

The following features are available in Busybox ash, so we don't need to
warn about them for runtime scripts:

- local keyword
- echo -n / -e
- String indexing

These warnings are new in shellcheck 0.7.2, which would otherwise fail
for various scripts.

[Matthias Schiffer: minor script improvements]