Duplicate a descriptor
#include <fd.h>
int fd_copy(to,from);
int to;
int from;
fd_copy copies descriptor from to descriptor to. If to was already open, fd_copy closes it. fd_copy always leaves from intact; if to and from are the same number, fd_copy does nothing.
fd_copy returns 0 on success, -1 on error. fd_copy does not guarantee that to will remain open, if it was open, in case of error.