Backport optional caching on all network streams.

Optionally allow caching for all networks streams, including those on LAN
(via new advanced setting 'alwaysforcebuffer')

As per xbmc commit https://github.com/xbmc/xbmc/commit/018fab22495e5de370b1e6b69e37e93657921f89

As discussed https://github.com/OpenELEC/OpenELEC.tv/pull/2531

Improves cache handling on Raspberry Pi's enourmously.

While I realize this will go ahead when gotham gets pulled down, the quality of life these changes make for Raspberry Pi, I'd greatly appreciate if these patches could be added meanwhile.