Change the key name for the libdispatch queue serial number from

"qserial" to "qserialnum" because "qserial" looks a lot like the
queue type (either 'serial' or 'concurrent') and can be confusing
to read through.  debugserver passes these up either in the questionmark
("T") packet, or in the response to the jThreadsInfo packet.

llvm-svn: 257121