Removed `goto fail` code with `unique_ptr`.

See merge request !3

Previously, `UDPTransport::SendMessageInternal` dynamically allocated a
`char[]` and used a `goto fail` to make sure that it was properly
deleted. Something like:

```c++
    char *buf = new char[100];
    if (...) {
        ...
        goto fail;
    } else if (...) {
        ...
        goto fail;
    } else {
        ...
    }

fail:
     delete [] buf;
     return false;
```

Now, the array is stored in a `unique_ptr` so that it's properly
deallocated when the function returns, without needing the goto fail.

A couple small lockserver client improvements.

1. Previously, typing ` `, `lock`, or `unlock` into a lockserver
   client's REPL would cause the client to crash because of some
   `strtok` calls that were returning `NULL`. Now, this bug is fixed;
   `strtok` returns are checked to be `NULL`, and no input should crash
   the client.
2. Previously, typing in an unrecognized command into the lockserver
   client's repl would print `Unknown command.. Try again!`. Now, it
   also prints out the set of legal commands `Usage: exit | q | lock
   <key> | unlock <key>`. This makes it a bit easier for someone
   tinkering around with TAPIR for the first time (like me) to know what
   to type.

fix building on Apple clang

This reverts commit 4791cfd3.

fixing one bug in TAPIR (not handling abstain) and one in IR, not correctly passing back decide result