Use better booleans for the proxy control variables

[matyasselmeci]

Just looking at whether *_DISABLE_HTTP_PROXY and *_DISABLE_PROXY_FALLBACK are set or unset is a pain to deal with, since it's sometimes hard to actually unset an environment variable (for example, in the docker command line).

IsProxyEnabled() and CanDisableProxy() should parse those environment variables as booleans using strconv.ParseBool(). Treat an invalid boolean (i.e. ParseBool returning an error) as false.

Update README.md to say that OSG_DISABLE_PROXY_FALLBACK must be set to true to be enabled.