Partially implement fork(2) (#892)

* 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).

* Move sys_fork to correct class; add a basic docstring

manticore/platforms/linux.py

@@ -1280,6 +1280,12 @@ class Linux(Platform):
 
         return len(data)
 
+    def sys_fork(self):
+        '''
+        We don't support forking, but do return a valid error code to client binary.
+        '''
+        return -errno.ENOSYS
+
     def sys_access(self, buf, mode):
         '''
         Checks real user's permissions for a file