string functions

This is on BPF tasks #574 as string factory (stringmap). Can we propose a minimal set of functions we'd like, to begin with? Here's my list:
- strstr() or strncmp() (or both)
- strlcat() and strlen(), or a basic snprintf()

Examples of current workarounds: strcmp():
- https://github.com/iovisor/bcc/blob/master/examples/networking/http_filter/http-parse-simple.c#L71
- https://github.com/weaveworks/scope/blob/master/examples/plugins/http-requests/http-requests.c#L95

strlcat():
- https://github.com/iovisor/bcc/blob/master/tools/execsnoop.py#L73

This emits arguments one by one. This is tricky, since we need to walk argv[] (currently using an unrolled loop). I was thinking strlcat() would let us build a concatenated args string, but it might need strlen() at some point. A basic snprintf() sounds much better, but .. gah this is awful.

How about a helper just for the argv[] case? :) Ie, take an argv[] argument and return a truncated string of the arguments. bpf_strarray_to_str().

Then, in lieu of another use case, my list is just:
- strstr() or strncmp() (or both)
- bpf_strarray_to_str()

@4ast may have another thread of discussion about this. I'm posting this here as it was suggested in IRC.

Start by reading BPF tasks #574 and the cited workarounds in http-parse-simple.c, http-requests.c, and execsnoop.py. The issue needs a decided minimal helper set—such as string comparison/search or argv[] conversion—before implementation can begin; done would mean an agreed scope and corresponding BPF string functionality.

Written by the indexing model from the issue text.