Merge branch 'ab/http-drop-old-curl-plus'

Merge branch 'ab/http-drop-old-curl-plus'

Conditional compilation around versions of libcURL has been
straightened out.

* ab/http-drop-old-curl-plus:
  http: don't hardcode the value of CURL_SOCKOPT_OK
  http: centralize the accounting of libcurl dependencies
  http: correct curl version check for CURLOPT_PINNEDPUBLICKEY
  http: correct version check for CURL_HTTP_VERSION_2
  http: drop support for curl < 7.18.0 (again)
  Makefile: drop support for curl < 7.9.8 (again)
  INSTALL: mention that we need libcurl 7.19.4 or newer to build
  INSTALL: reword and copy-edit the "libcurl" section
  INSTALL: don't mention the "curl" executable at all