Various additions and fixes.

src/lib9/exec.c

@@ -0,0 +1,9 @@
+#include <u.h>
+#include <libc.h>
+
+int
+exec(char *prog, char *argv[])
+{
+	/* to mimic plan 9 should be just exec, but execvp is a better fit for unix */
+	return execvp(prog, argv);
+}