client API: allow returning float properties as integers

I'm starting to think that being type-strict with this interface
actually sucks. This commit is a step towards being less strict.
osd: properly redraw external overlays when they are set
stream_lavf: expose concat://

Apparently there's an use for this; see #1178.

I won't redocument obscure FFmpeg features, so add a hint to the
manpage that some protocols are documented in FFmpeg instead.
player: fix OSD cycling

OSD cycling attempted to remove the current message by setting an empty
message with duration 0. Duration 0 tripped up a corner case causing no
OSD to be displayed (until the next message was set), so exclude this
explicitly.