stream_file: properly detect stdin as pipe

There is some code that checks a FD for whether it is a regular file or
not. If it's not a regular file, it e.g. enables use of poll() to avoid
blocking forever.

But this was done only for FDs that were open()ed by us, not from stdin
special handling or fd://. Consequently, " | mpv -" could block the
player. Fix this by moving the code and running for it on all FDs.

Also, set p->regular_file even on mingw.