Allow user.dat fields: DLB (downloaded-bytes), ULB (uploaded-bytes), and CDT

(credits) to use the full 4 gibibyte range of a 32-bit unsigned integer:
max-out at 4,294,967,295 instead of 2,147,483,647.
Since these values are stored as 10 characters, we could theoretically extend
the range to 9,999,999,999 (credits/bytes) without changing the user.dat format
but is a bigger change (moving to 64-bit integers everywhere these values are
read or adjusted).