avformat/flvenc: add no_sequence_end flags for flvflags

when split flv file by flv format at first, and cat flvs file
into one flv file, the flv sequence end is be used,
then the whole flv have many flv sequence end TAG.
this flags can give user an option to ignore write sequence end TAG

Signed-off-by: Steven Liu <lingjiujianke@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>