don't arbitrarily limit UIDs to a billion

the number was chosen to make queries more comprehensible when the
server sends no UIDNEXT, but it appears that such insanely large UIDs
actually show up in the wild. so send 32-bit INT_MAX instead.

note that this is again making an assumption: that no server uses
unsigned ints for UIDs. but we can't sent UINT_MAX, as that would break
with servers which use signed ints. also, *we* use signed ints (which is
actually a clear violation of the spec).

it would be possible to special-case the range [1,inf] to 1:*, thus
entirely removing arbitrary limits. however, when the range doesn't
start at 1, we may actually get a single message instead of none due to
the imap uid range limits being unordered. this gets really nasty when
we need to issue multiple queries, as we may list the same message
twice.

a reliable way around this would be issuing a separate query to find the
actual value of UID '*', to make up for the server not sending UIDNEXT
in the first place. this would obviously imply an additional round-trip
per mailbox ...

src/drv_imap.c

@@ -2300,7 +2300,7 @@ imap_load_box( store_t *gctx, int minuid, int maxuid, int newuid, int *excs, int
 			imap_submit_load( ctx, buf, 0, sts );
 		}
 		if (maxuid == INT_MAX)
-			maxuid = ctx->gen.uidnext ? ctx->gen.uidnext - 1 : 1000000000;
+			maxuid = ctx->gen.uidnext ? ctx->gen.uidnext - 1 : 0x7fffffff;
 		if (maxuid >= minuid) {
 			if ((ctx->gen.opts & OPEN_FIND) && minuid < newuid) {
 				sprintf( buf, "%d:%d", minuid, newuid - 1 );