[WIP] Fix most warnings generated from -Wmost, for SimTKcommon.

Addresses #181.

See travis output for the remaining warnings. So far, I only addressed warnings from SimTKcommon. SimTKcommon still has two warnings, but I don't know what to do with them.
This is what caused the TestPlugin failure. `getcwd()` must be called no matter what -- it writes the current working directory into `buf`. It is just that its return value is only checked in Debug. So the code could read something like:
```
#ifndef NDEBUG
    char* bufp = getcwd(buf, sizeof(buf));
    assert(bufp);
#else
    (void) getcwd(buf, sizeof(buf))
#endif
```
This is one of those cases where fixing the warnings actually makes the code uglier.