Add an attribute to start checking format strings
fix warnings now noted due to format string checking
piano: Request track length

The returned length is used as first length estimate before enough audio
data arrives to show the real length and for the eventcmd API. See #427
and #64.
waitress: Increase kernel receive buffer

Suggested in #426. Should be large enough for a complete song now (if
permitted by sysctl). The correct solution™ would be a buffer in
userspace though.
Merge branch 'master' of https://github.com/PromyLOPh/pianobar