add libc scheme for /dev/tty etc.

Filenames beginning with libc: will be rerouted to libcscheme::open. Currently, libc:{tty,stdin,stdout,stderr} are implemented. It is expected that there will be symbolic links from the file: scheme, e.g. file:/dev/tty -> libc:tty. This should probably be done in the filesystem config for now. libc:tty is implemented by reading the $TTY environment variable and opening the corresponding device scheme. The file descriptor returned by the libc: scheme is that of the device, so the libc: scheme does not need to implement anything other than open.