Made demux2_IsPathExtension case insensitive.
Used demux2_IsPathExtension/p_demux->b_force when appropriate
Fixed a regression in flac demuxer (segfault when reading meta data)
Added a VLC_UNUSED(x) macro to avoid seeing ugly (void)x around the code.
Improved a bit a few vlc_common.h macro (do {} while(0))