[PATCH 0/2] tolower cleanups

These two patches were pulled from the http charset series I posted nearby. The second iteration of that series did not need them, but they may have value as cleanups.

  [1/2]: daemon/config: factor out duplicate xstrdup_tolower
  [2/2]: strbuf: add strbuf_tolower function

The first one is a real reduction of code. The second is just making code public that _might_ get used later, so it's a bit less exciting. Both are independent of each other, so we can take or leave them separately.