shairport: Improve init script.
- Switch to procd.
- Fix alsa options.
- Fix indentation.
- Fix white spaces not being accepted on config values.
- Workaround procd missing custom signal.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Merge pull request #116 from Noltari/shairport-next

shairport: Improve init script.