avformat/hlsenc: enhanced %v handling with variant names

avformat/hlsenc: enhanced %v handling with variant names

When multiple variant streams are specified by var_stream_map option, %v
placeholder in various names ensures that each variant has its unique
names. Currently %v is substituted by its variant index value (0, 1, 2
etc.). In some use cases it would be handy to specify names for variants
instead of numerical indexes. This patch makes it possible to use names
instead of default indexes. In var_stream_map option each or some of the
variant streams may use an optional name attributum (e.g.
-var_stream_map "v:0,a:0,name:sd v:1,a:1,name:720p") If a name is
specified for a variant, then this name value will be used as
substitution value of %v instead of the default index value.

Signed-off-by: Bela Bodecs <bodecsb@vivanet.hu>
Signed-off-by: Steven Liu <lq@onvideo.cn>