fileio: introduce take_fdopen{_unlocked}() variant

fileio: introduce take_fdopen{_unlocked}() variant

With the addition of _cleanup_close_ there's a repetitious
pattern of assigning -1 to the fd after a successful fdopen
to prevent its close on cleanup now that the FILE * owns the
fd.

This introduces a wrapper that instead takes a pointer to the
fd being opened, and always overwrites the fd with -1 on success.

A future commit will cleanup all the fdopen call sites to use the
wrapper and elide the manual -1 fd assignment.