BPF pre-processor for command line options

Tools currently use string substitution to implement different BPF code for different command line options. We're pushing it a bit too far in some tools, eg, opensnoop:

```Python
PID_TID_FILTER
UID_FILTER
FLAGS_FILTER
[...]
if args.tid: # TID trumps PID
bpf_text = bpf_text.replace('PID_TID_FILTER',
'if (tid != %s) { return 0; }' % args.tid)
elif args.pid:
bpf_text = bpf_text.replace('PID_TID_FILTER',
'if (pid != %s) { return 0; }' % args.pid)
else:
bpf_text = bpf_text.replace('PID_TID_FILTER', '')
if args.uid:
bpf_text = bpf_text.replace('UID_FILTER',
'if (uid != %s) { return 0; }' % args.uid)
else:
bpf_text = bpf_text.replace('UID_FILTER', '')
if args.flag_filter:
bpf_text = bpf_text.replace('FLAGS_FILTER',
'if (!(flags & %d)) { return 0; }' % flag_filter_mask)
else:
bpf_text = bpf_text.replace('FLAGS_FILTER', '')
if not (args.extended_fields or args.flag_filter):
bpf_text = '\n'.join(x for x in bpf_text.split('\n')
if 'EXTENDED_STRUCT_MEMBER' not in x)
```

This wasn't too bad for simple PID filters, but is getting messier, especially if we start having more -e extended field modes, where we want to have a perf output struct that also varies based on command line options. This ticket is to discuss a better way.

@vincentbernat once suggested (3 years ago) that I look at jinja2. I don't know it, but it looks interesting: https://github.com/vincentbernat/systemtap-cookbook/blob/116500e65c42fe2f9b5cea4f42961db9e26685b9/io#L239-L315

Resulting code has things like:

```
{%- if options.milliseconds %}
delta /= 1000;
{%- endif %}
```

Any other such libraries?

I think someone needs to pick a tool (say, opensnoop), and convert it to use these libraries so we can see if it's an improvement or not.