[cmyth] Force LiveTV chain update in done_recording

- Backend could return old program during chain update. When old recording is completed and the chain has not been updated during normal flow, i retry update 4 times in "done_recording" before returning error. That is a workaround against this abnormal case using Myth protocol. An other fix could be use DB (tvchain) but need huge rework.
- Multi players: Call "get_cur_proginfo" only when chainid matches.
[cmyth] Resolve LiveTV chain seek

- cmyth_file_seek: Fix read position to the position returned by backend.
- cmyth_livetv_chain_seek: Force SEEK_SET because read position could not be the same when switching to an other file. Returns absolute position in the chain.
[cmyth] Switch on read does'nt reset the file position to 0

- When next file in the chain have been already requested, playback continues from the last position. So after switching file during playback, we seek to position 0.