Draft: Add SYS_FRETURNFD, xopen, and xdup.
Adds the scheme "response code" SYS_FRETURNFD (not a syscall, but used similar to SYS_FEVENT when it acts as a message to the kernel scheme socket), and adds the extended functions xopen and xdup to the Scheme and SchemeMut traits. These return OpenedFile, which allows either scheme-local or cross-scheme returned file descriptors.