It includes various Windows-specific headers when compiling
for Windows and these sometimes cause issues: E.g. winbase.h
defines IGNORE, which clashes with a macro used in the Matroska
muxer (since 884653ee) and demuxer.

This header provides fallback defines for various stuff that is
mostly not used directly by (de)muxers at all:
mkdir, rename, rmdir, unlink, access, poll, pollfd, nfds_t,
closesocket, socklen_t, fstat, stat, lseek, SHUT_(RD|WR|RDWR)
and various POLL* constants.

Ergo fix this issue by not auto-including this header in lots
of places via an inclusion in internal.h and instead include
it everywhere where the above stuff is used (most of these
translation units already included os_support.h).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>