avconv: make -shortest work with streamcopy

CC: libav-stable@libav.org
(cherry picked from commit 48e50921337984ba4ec2c1cafe45d43787f84498)

Signed-off-by: Anton Khirnov <anton@khirnov.net>
avconv: do not send non-monotonous DTS to the muxers.

Hack partially based on a commit by Michael Niedermayer <michaelni@gmx.at>
Should fix (or work around) bug 458.
(cherry picked from commit 76d23f40314fc1dcd74a3d470b17782cc0ee5a3a)

Signed-off-by: Anton Khirnov <anton@khirnov.net>