ismindex: produce .ismf file

This is a non-standard file that maps the MSS segment names to offsets
in the ISMV file. This can be used to build a custom MSS streaming
server without splitting the ISMV into separate files.

Signed-off-by: Martin Storsjö <martin@martin.st>
get_bits: Add OPEN_READER macro variant w/o size_plus8

This avoids a trillion warnings from MSVC.
rdft: Move some variables into a separate block

This avoids an unused variable warning with hardcoded tables.
Replace av_unused attributes by block structures

This is more portable and avoids warnings with compilers that do not
properly support av_unused.
Drop unnecessary av_unused attributes.
avplay: Exit by default at the end of playback

This is the expected behavior shared by all other cli multimedia players.

Bug-Id: 732