fd-util: add new helper fd_reopen_conditional()

fd-util: add new helper fd_reopen_conditional()

This is a wrapper around fd_reopen() that will reopen an fd if the
F_GETFL flags indicate this is necessary, and otherwise not.

This is useful for various utility calls that shall be able to operate
on O_PATH and without it, and might need to convert between the two
depending on what's passed in.