Partially implement fork(2)

Really this return ENOSYS (not implemented), which allows sub-processes to
detect the condition and continue to run (or crash on their own if they did not
expect fork() to fail).

manticore/platforms/linux.py

@@ -2621,6 +2621,9 @@ class SLinux(Linux):
 
         return super(SLinux, self).sys_getrandom(buf, size, flags)
 
+    def sys_fork(self):
+        return -errno.ENOSYS
+
     def generate_workspace_files(self):
         def solve_to_fd(data, fd):
             try: