Read a 4-byte mtime as unsigned (old-protocol).

When conversing with a protocol 29 or earlier rsync, the modtime values
are arriving as 4-byte integers.  This change interprets these short
values as unsigned integers, allowing the time that can be conveyed to
range from 1-Jan-1970 to 7-Feb-2106 instead of the signed range of
13-Dec-1901 to 19-Jan-2038.  Given that we are fast approaching 2038,
any old-protocol transfers will be better served using the unsigned
range rather than the signed.

It is important to keep in mind that protocol 30 & 31 convey the full
8-byte mtime value (plus nanoseconds), allowing for a huge span of time
that is not affected by this change.

flist.c

@@ -836,7 +836,7 @@ static struct file_struct *recv_file_entry(int f, struct file_list *flist, int x
 			}
 #endif
 		} else
-			modtime = read_int(f);
+			modtime = read_uint(f);
 	}
 	if (xflags & XMIT_MOD_NSEC)
 #ifndef CAN_SET_NSEC

io.c

@@ -1784,6 +1784,13 @@ int32 read_int(int f)
 	return num;
 }
 
+uint32 read_uint(int f)
+{
+	char b[4];
+	read_buf(f, b, 4);
+	return IVAL(b, 0);
+}
+
 int32 read_varint(int f)
 {
 	union {