libavformat/segment.c: Add strftime expansion for segment filename templates

Allows expansion of the filename template with strftime() with the option
-strftime 1 (disabled by default). This allows segments to be named by time of
creation, adding some flexibility.

Fixes Ticket 4104 (add strftime to segment muxer)

Signed-off-by: Pedro E. M. Brito <pedroembrito@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>