Y2K38: the 2038 problem

[Zankaria]

Is vichan 2038 ready? I know the issue is still somewhat far away, but given how long the codebase has managed to survive for we might as well look at it.
What must be checked:

• The PHP codebase. Should be fine, mostly? At least as long as it's run on a 64-bit CPU.
• The SQL schema: we're using unsigned ints, are they safe?
• The SQL queries. We should review the SQL functions in particular, as things as UNIX_TIMESTAMP() return a 32-bit value.