Fix: now mp4 file on internet url (http, etc) can be played by dvdplayer.

currently in frodo, mp4 file on on http link is unplayable, after some dig, I fount it's because the dvdplayer(ffmpeg/avformat) handle mp4 file with many many random seeks between file head, middle and end. then check the file cache code, the cache will be reset after each seek, it cause the file is totally unplayable by xbmc.
by check other players, I found:
in vlc, it almost same with xbmc, almost unplayable
in mplayer, it cache from file head until the seek position then continue it can play
in ios native player, it also seeks from here to there, but it seems has a better cache strategy or keep the main stream continue cached, it also played well.

so, an easy fix for this type file (frequently random seek), cache should work in different strategy.

this commit fix this by set frequent_random_seek flag to cache let cache work in the mplayer way, then the mp4 file on net url can be played by xbmc.