stap: wait for stap process to quit
tcp: distribution of accept queue latencies

This is a copy of agentzh version with a slight modification to make
it work with a blocking accept().
stap: document supported Linux versions
tcp: monitor TCP syn/ack queue overflow
profile: profiling backtraces

For user backtrace, a PID should be specified. This makes the script a
little less useful that it could be.
io: add something like iotop
io: display average request sizes
io: allow to group top by command instead of by PID
io: allow to sort top by other metrics