Backporting PR 2843

Optionally allow caching for all network streams, including those on LAN (via new advanced setting 'alwaysforcebuffer')

Conflicts:
	xbmc/cores/dvdplayer/DVDInputStreams/DVDInputStreamFile.cpp
	xbmc/settings/AdvancedSettings.cpp
	xbmc/settings/AdvancedSettings.h