piano: Generic linked lists

Introduces generic linked list structure and functions (like append,
delete, …). Removes a lot of copy&pasted code and improves code
readability/reusability.

Heads up: This change breaks libpiano’s ABI.
waitress: Try all addresses returned by getaddrinfo

Fixes issue #384.
Merge remote-tracking branch 'promy/master'
Merge remote-tracking branch 'promy/master' into dl
Remove nonexisting list.h from Makefile

Oops. Fixes #386.
headless_pianobar: Support command-line arguments

See issue #383.
Merge remote-tracking branch 'promy/master'
Merge branch 'master' into dl