Adds a new sample client to show how drmgr's opcode instrumentation can be used to count the number of executions of an instruction (with a specific opcode). The opcode number is given by the user as a parameter (the client uses droptions for parsing).
In addition, the PR also includes a bug fix in drmgr. In particular, drmgr was confusing the default priority as a different priority when used again for another event registration. The bug was only triggered if cblist_insert_other() is used - this function is only called for opcode registration at the moment so the bug's severity is low.