rvm-installer still believes it's performing a multi-user install when installing for root only

I followed the instructions in section "I want to install for root only!" of the FAQ (see https://rvm.io/support/faq/), and rvm-installer still proceeded as if it was doing a system install in a couple of ways. Most importantly, it lays down an `/etc/rvmrc` file that forces `rvm` to use a umask that gives write permissions to the group.

It seems like we need to do something closer to what `scripts/rvm` already does for determining the value of the `$rvm_user_install_flag`.