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