avformat/dashdec: add dash demuxer base version

ffmpeg need a dash demuxer for demux the dash formats base on
https://github.com/samsamsam-iptvplayer/exteplayer3/blob/master/tmp/ffmpeg/patches/3.2.2/000001_add_dash_demux.patch

TODO:
1. support multi bitrate dash.

v2 fixed:
1. from autodetect to disabled
2. from camelCase code style to ffmpeg code style
3. from RepType to AVMediaType
4. fix variable typo
5. change time value from uint32_t to uint64_t
6. removed be used once API
7. change 'time(NULL)`, except it is not 2038-safe.' to av_gettime and av_timegm
8. merge complex free operation to free_fragment
9. use API from snprintf to av_asprintf

v3 fixed:
1. fix typo from --enabled-xml2 to --enable-xml2

v4 fixed:
1. from --enable-xml2 to --enable-libxml2
2. move system includes to top
3. remove nouse includes
4. rename enum name
5. add a trailing comma for the last entry enum
6. fix comment typo
7. add const to DASHContext class front
8. check sscanf if return arguments and give warning message when error
9. check validity before free seg->url and seg
10. check if the val is null, before use atoll

v5 fixed:
1. fix typo from mainifest to manifest

v6 fixed:
1. from realloc to av_realloc
2. from free to av_free

v7 fixed:
1. remove the -lxml2 from configure when require_pkg_config

v8 fixed:
1. fix replace filename template by av_asprintf secure problem

v9 modified:
1. make manifest parser clearly

v10 fixed:
1. fix function API name code style
2. remove redundant strreplace call
3. remove redundant memory operation and check return value from get_content_url()
4. add space between ) and {
5. remove no need to log the value for print

v11 fixed:
1. from atoll to strtoll
Suggested-by: Michael Niedermayer <michael@niedermayer.cc>

v12 fixed:
1. remove strreplace and instead by av_strreplace
Suggested-by: Nicolas George <george@nsup.org>

v13 fixed:
1. fix bug: cannot play:
http://dash.edgesuite.net/akamai/bbb_30fps/bbb_30fps.mpd
Reported-by: Andy Furniss <adf.lists@gmail.com>

v14 fixed:
1. fix bug: TLS connection was non-properly terminated
2. fix bug: No trailing CRLF found in HTTP header
Reported-by: Andy Furniss <adf.lists@gmail.com>

v15 fixed:
1. play youtube link: ffmpeg -i $(youtube-dl -J "https://www.youtube.com/watch?v=XmL19DOP_Ls" | jq -r ".requested_formats[0].manifest_url")
2. code refine for timeline living stream
Reported-by: Ricardo Constantino <wiiaboo@gmail.com>

v16 fixed:
1. remove the snprintf and instead by get_segment_filename make safety
2. remove unnecessary loops
3. updated xmlStrcmp and xmlFree to av_* functions
4. merge code repeat into one function
5. add memory alloc faild check
6. update update_init_section and open_url
7. output safety error message when filename template not safe
Suggested-by : wm4 <nfxjfg@googlemail.com>

v17 fixed:
1. add memory alloc faild check
2. fix resource space error at free_representation

v18 fixed:
1. add condition of template format

v19 fixed:
1. fix typo of the option describe

v20 fixed:
1. add the c->base_url alloc check
2. make the DASHTmplId same to dashenc

v21 fixed:
1. remove get_repl_pattern_and_format and get_segment_filename
2. process use dashcomm APIs

v22 fixed:
1. modify the include "dashcomm.h" to include "dash.h"
2. use internal API from dash_fill_tmpl_params to ff_dash_fill_tmpl_params

Signed-off-by: Steven Liu <lq@onvideo.cn>
Signed-off-by: samsamsam <samsamsam@o2.pl>
