pub_tool_oset.h and m_oset.c cleanup

* Remove dead code in m_oset.c VG_(OSetGen_ResetIterAt)
  The code at the end of VG_(OSetGen_ResetIterAt) was unreachable
  (detected by BEAM checker).
  Looking at SVN, the initial commit of VG_(OSetGen_ResetIterAt)
  already contained this deadcode.

* pub_tool_oset.h was wrongly indicating that signed words could
  be used for fast cmp oset.

* modified memcheck/tests/unit_oset.c to test VG_(OSetGen_ResetIterAt)

* modified memcheck/tests/unit_oset.c to not use signed words
  (it was previously using signed words, but only with positive values)





git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13622 a5019735-40e9-0310-863c-91ae7b9d1cf9