Add new module "pam_escalate.so" for non-root applications

For issue #7, I'm adding a new module "pam_escalate.so" that proxies authentication requests to another PAM stack in a setuid-root helper called pam-escalate-helper. Along with proxying the authentication conversation, it does some scrubbing and security checks to make sure that the caller only tries to do auth for their own user unless the caller is root.

A typical workflow would be:

1. An application running as a normal user, like gnome-screensaver, calls pam_start and pam_authenticate.
2. PAM loads pam_escalate.so and calls pam_sm_authenticate.
3. pam_escalate.so launches the subprocess pam-escalate-helper and sends a start message to its stdin.
4. Helper calls pam_start with some args from the start message and its own conversation function.
6. For each call to the helper's conversation module by other PAM modules like pam_krb5, it sends the message over stdout and receives a response from pam_escalate.so on stdin.
7. When pam_authenticate() finishes, the helper sends a finish message with the return code and exits.
8. pam_escalate.so receives the finish message and returns the code to the user.

I'm not done yet, but here's an implementation that actually works. I still need to add more comments, more unit-tests, and probably better security checks. I should have it mostly finished tomorrow, but I figured I'd send the first parts early.

At some point I'll break it out into another project, but I'm just trying to get this done quickly for now.
Sorry, I 'm not sure I understand. The type is contained expected_str since it has an entire serialized message. Do you want me to construct the message instead?

For example, if the given expected_str is ```"(2, <(1, 'Password: ')>)"``` then it is type 2 (prompt).

The entire serialized message is given in expected_str so I don't have to make a ton of code to construct a message. I figured that was cleanest, but I'm happy to do it another way. This test code got insane kinda fast. :/