Refactor IP subnet comparison using apr funcs

* comparison of connecting IP against trusted RPAF_ProxyIPs done with
  apr_ipsubnet_test() rather than previous approach that relied on
  ipv4-limited check_cidr() and strcmp()

* rpaf_looks_like_ip() checks that RPAF_Header request values and
  RPAF_ProxyIPs config values contain valid ipv4/6 addresses

* new RPAF_ForbidIfNotProxy option to forbid requests not connecting
  from RPAF_ProxyIps; otherwise difficult to use Allow/Deny access
  control on proxies after remote address substitution has taken place

* ifdef brackets around code dealing with server_rec->server_scheme
  because that struct member is only in versions >= 2.2.3

* set "remoteip-proxy-ip-list" value in r->notes table to list of
  proxies so other modules can know about trusted intermediaries and
  IP substitution; similar to Apache2.4 mod_remoteip