Get dbgen compiling (warning-free) with GCC 4.6.

Summary:
Several of these are fixing actual bugs (sprintf'ing into a FILE*,
sprintf'ing into a char*[], printf'ing an int with %lld or vice
versa). Some of them are a little fiddly given that "long" and
"long long" are both 64 bits on all the platforms we currently
care about.

Also add an .arcconfig and .gitignore for arcanist-friendliness,
and git rm the OSX metadata file ".DS_Store" in two places.

Test Plan: psyduck

Reviewed By: nikita, chase

Differential Revision: http://grizzly.memsql.com/D3846