Merge branch 'tb/http-curl-keepalive'

TCP keepalive behaviour on http transports can now be configured by calling cURL library. * tb/http-curl-keepalive: http.c: allow custom TCP keepalive behavior via config http.c: inline `set_curl_keepalive()` http.c: introduce `set_long_from_env()` for convenience http.c: remove unnecessary casts to long