Basically, it's a conflict between POSIX and ISO C what do to when
input streams are passed to fflush().
POSIX mandates that the seeking-position should be synced, but ISO C
says it's undefined behaviour.
We love POSIX, but the standard-documents specify that in all conflict
cases, ISO C wins, so this breaks with EBADF on BSD's.
musl and glibc follow POSIX behaviour, which makes sense, but involves
numerous portability concerns.
To get around this, we just don't check fflush() and rely on the fact
that no implementation sets ferror on the file-stream in fflush if it
is an input stream, so every issue caught in fflush() is caught later
with ferror() and fclose().
Add a comment to fshut() because this stuff is so complicated, it
took us a day to figure out.
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