Code clean-up to use size_t/ssize_t when they are the right type.

* jk/size-t:
  strbuf_humanise: use unsigned variables
  pass st.st_size as hint for strbuf_readlink()
  strbuf_readlink: use ssize_t
  strbuf: use size_t for length in intermediate variables
  reencode_string: use size_t for string lengths
  reencode_string: use st_add/st_mult helpers