fd_move - renumber a descriptor

#include <fd.h>

int fd_move(to,from);

int to;
int from;

fd_move moves descriptor from to descriptor to. If to was already open, fd_move closes it. If the move is successful, fd_move closes from. Exception: if to and from are the same number, fd_move does nothing.

fd_move returns 0 on success, -1 on error.

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