Fix ftplugin files not being entered in some buffers

Fixes `maktaba#plugin#Enter` to use `bufnr('%')` instead of `bufnr('.')` to get the correct number for the current buffer. The guard logic for ftplugin files was getting duplicate buffer numbers and sometimes preventing ftplugin files from being entered.

Fixes #75.
Also fixes test failures in plugincontrol.vroom.