build: remove BSD-specific /usr/local include path additions

It seems it's generally cleaner to leave this stuff to the BSDs.
Additionally, the NetBSD part wasn't even correct, because it made the
compiler prefer the system include path before local include files.
manpage: remove some trailing whitespace

I wish I could make github run a hook to reject these.
manpage: explicitly document protocols
DOCS: remove en/ sub-directory

This additional sub-directory doesn't serve any purpose anymore. Get rid
of it.
vo_xv: de-ancient some help messages

One message pointed to a removed file, so just get rid of the messages.
They were helpful in the earlier 2000s, but now they're just confusing.
stream: minor cleanups

Remove unused stream type constants. Move some now DVD specific crap
to stream_dvd.c.
stream: add a file cache

For remarks, pretty much see the manpage additions. Could help with
network streams that require too much seeking (maybe), or might be
extended to help with the use case of watching and downloading a file
at the same time.

In general, it might be a useless feature and could be removed again.