Add a C shell, which currently just passes through

This is a first step toward moving the UI to C and
adding file and network I/O in C. The fractal core
will remain in assembler as well as the multiplier.

mandel.h

@@ -0,0 +1,4 @@
+#include <inttypes.h>
+
+// From mandel-core.s:
+extern void mandel_start(void);