Propagate the error messages from the webserver better to the
client coming over the HTTP transport.

* jk/http-errors:
  http: default text charset to iso-8859-1
  remote-curl: reencode http error messages
  strbuf: add strbuf_reencode helper
  http: optionally extract charset parameter from content-type
  http: extract type/subtype portion of content-type
  t5550: test display of remote http error messages
  t/lib-httpd: use write_script to copy CGI scripts
  test-lib: preserve GIT_CURL_VERBOSE from the environment