Fix leaking memory when daemonizing falcond

Summary:
As said in the C++ documentation about exit (
[[http://www.cplusplus.com/reference/cstdlib/exit/| cpluplus reference]]):

"objects with automatic storage are not destroyed by calling exit (C++)".

So, using _exit (man 2) or exit (man 3) won't call the object destructors.

Our actual design won't need to use "exit" function. So use return instead.

Test Plan: valgrind --leak-check=full ./falcond --daemon

Reviewers: adrienconrath

Reviewed By: adrienconrath

Subscribers: NicolasDP

Differential Revision: http://91.121.199.76/D57