Move UID/UIDVALIDITY/SeqNum from signed 32 -> unsigned 32: Bug #737642

Prior code inadvertantly limited UID and UIDVALIDITY to signed, not
unsigned, 32-bit integers.  I've also added stronger checking of
numbers received off the wire, both that they're truly numeric
according to RFC and that they're within boundaries, if specified.

This also enforces bit width of integers from IMAP string parameters
to avoid similar problems in the future.