Pushed several GCC-related fixes and warning suppression commits

[Ben Pfaff]

I recently upgraded my main dev machine from Debian "squeeze" to
"wheezy" and part of that was upgrading from GCC 4.3 to GCC 4.7.
The newer version of GCC has better or sometimes just different
warnings, so I spent some time cleaning up the PSPP code to fix
the bugs that it found and otherwise suppress the harmless
warnings.

Please look over the commits and let me know if any of them seem
wrong.

Thanks,

Ben.