avformat/hlsenc: support multi level path in m3u8 with filename

avformat/hlsenc: support multi level path in m3u8 with filename

before patch:

localhost:osx liuqi$ tree 20160926/
20160926/
└── file-20160909
    └── 26
        └── 16
            └── 15
                ├── 46
                │   ├── 13.ts
                │   ├── 25.ts
                │   ├── 36.ts
                │   ├── 45.ts
                │   └── 54.ts
                └── 47
                    ├── 04.ts
                    ├── 14.ts
                    ├── 24.ts
                    └── 35.ts

6 directories, 9 files
localhost:osx liuqi$ cat out.m3u8
 #EXTM3U
 #EXT-X-VERSION:3
 #EXT-X-TARGETDURATION:12
 #EXT-X-MEDIA-SEQUENCE:0
 #EXTINF:11.360000,
 13.ts
 #EXTINF:10.800000,
 25.ts
 #EXTINF:9.120000,
 36.ts
 #EXTINF:8.760000,
 45.ts
 #EXTINF:10.200000,
 54.ts
 #EXTINF:10.720000,
 04.ts
 #EXTINF:9.600000,
 14.ts
 #EXTINF:10.600000,
 24.ts
 #EXTINF:8.760000,
 35.ts
 #EXT-X-ENDLIST

after patch:

localhost:osx liuqi$ tree 20160926/
20160926/
└── file-20160909
    └── 26
        └── 16
            └── 15
                ├── 46
                │   ├── 13.ts
                │   ├── 25.ts
                │   ├── 36.ts
                │   ├── 45.ts
                │   └── 54.ts
                └── 47
                    ├── 04.ts
                    ├── 14.ts
                    ├── 24.ts
                    └── 35.ts

6 directories, 9 files
localhost:osx liuqi$ cat out.m3u8
 #EXTM3U
 #EXT-X-VERSION:3
 #EXT-X-TARGETDURATION:12
 #EXT-X-MEDIA-SEQUENCE:0
 #EXTINF:11.360000,
 20160926/file-20160909/26/16/15/46/13.ts
 #EXTINF:10.800000,
 20160926/file-20160909/26/16/15/46/25.ts
 #EXTINF:9.120000,
 20160926/file-20160909/26/16/15/46/36.ts
 #EXTINF:8.760000,
 20160926/file-20160909/26/16/15/46/45.ts
 #EXTINF:10.200000,
 20160926/file-20160909/26/16/15/46/54.ts
 #EXTINF:10.720000,
 20160926/file-20160909/26/16/15/47/04.ts
 #EXTINF:9.600000,
 20160926/file-20160909/26/16/15/47/14.ts
 #EXTINF:10.600000,
 20160926/file-20160909/26/16/15/47/24.ts
 #EXTINF:8.760000,
 20160926/file-20160909/26/16/15/47/35.ts
 #EXT-X-ENDLIST

Signed-off-by: Steven Liu <lingjiujianke@gmail.com>