[PATCH] Make SBuf::find_first_of O(N) and implement find_first_not_of

Hi,
  attached patch is from branch lp:~squid/squid/characterset/

It extracts some of the work done in parser-ng to implement Alex'
Tokenizer API, bringing O(N) find-any{,-not}-of matching to SBuf.

Note: in IRC discussions, Amos mentioned find_first_of and
find_first_not_of not being naming convention compliant (that's right,
they are modeled after std::string). I'm fine with changing them as a
follow-up commit if this is the collegial decision; I'd avoid do to it
as part of this merge.

-- 
    /kinkie