Added gdb-remote expedited register dupe check.

The llgs branch had a bug where register sets were not terminated with
LLDB_INVALID_REGNUM so the expedite register loop was issuing duplicate
registers.  This test was added to catch the problem.

Enhanced the key-val collection method to optionally (and by default)
support capturing duplicate values for a given key.  When that happens
and if permitted, it promotes a single key to a list and appends values
to it.