Issue a warning when int('0...', 0) returns an int with the sign

Issue a warning when int('0...', 0) returns an int with the sign
folded; this will change in Python 2.4.  On a 32-bit machine, this
happens for 0x80000000 through 0xffffffff, and for octal constants in
the same value range.  No warning is issued if an explicit base is
given, *or* if the string contains a sign (since in those cases no
sign folding ever happens).