Added dr_where_am_i() to better support client self-profiling via sampling. This also provides the fragment tag, by refactoring the pcprofile code into a new helper fcache_refine_whereami().
Renamed the whereami types to better avoid name conflicts: s/WHERE_xxx/DR_WHERE_xxx/ and s/where_am_i_t/dr_where_am_i_t/. Exported the dr_where_am_i_t enum.
Fixed a bug where an uninitialized mcontext was passed to a client timer callback: we only filled in the mcontext for a DR-internal callback.
Added a test of client sampling to client.timer
Issue: #140