Previously, map.lookup_or_init() may cause unexpected
return from the function when lookup finds no element and
init failed e.g. due to unlikely racy update or
sometimes hash table full.
This has caught surprise from many users. So, the commit
  https://github.com/iovisor/bcc/commit/ba64f031f2435aad5a85f8f37dbbe2a982cbbe6b
attempts to remove the early return in map.lookup_or_init().
But then since NULL pointer could be returned,
user will need to change their bpf program to check return value,
otherwise, verifier will reject the program.

As described in the above, such an API behavior change may cause
verifier failure and reject previously loadable bpf programs.
bcc should try to maintain API stability, esp. to avoid subtle
API behavior change.

This patch propose to restore the behavior of map.lookup_or_init()
and introduce a new one map.lookup_or_try_init(), which will
avoid unexpected return. The name is suggested by Alexei
to reflect that init may fail. map.lookup_or_try_init() will be formally
documented and used in bcc. A warning will be generated if
map.lookup_or_init() is used. Documentation will make it clear
that map.lookup_or_try_init() is preferred over map.lookup_or_init().

```
-bash-4.4$ sudo ./syscount.py
/virtual/main.c:71:11: warning: lookup_or_init() may return from the function, use loopup_or_try_init() instead.
    val = data.lookup_or_init(&key, &zero);
          ^
1 warning generated.
Tracing syscalls, printing top 10... Ctrl+C to quit.
...
```

All uses in examples and tools are converted to use
lookup_or_try_init(). Most tests are converted to use
lookup_or_try_init() too except test_trace_maxactive.py
and test_tracepoint.py to test lookup_or_init()
functionality.