http: fix EVHTTP_CON_READ_ON_WRITE_ERROR when it doesn't supported by OS For example win32 doesn't accept such things (maybe via overloaded IO, I'm not sure), also I looked into curl and seems that the behaviour is the same (IOW like with EVHTTP_CON_READ_ON_WRITE_ERROR on linux/win32). Fixes: https://ci.appveyor.com/project/nmathewson/libevent/build/2.1.5.216#L499 (win32) Fixes: 680742e1665b85487f10c0ef3df021e3b8e98634 ("http: read server response even after server closed the connection") v2: v0 was just removing that flag, i.e. make it deprecated and set_flags() will return -1