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