Reorder to make priotities match with xdg-open for xdg-settings generic (fixes #266)

Please review, I tested this one, ran shellcheck, but I was kind of tired while writing, more eyes on the code can't hurt.

for the generic case this patch makes sure xdg-settings doesn't turn useless when the BROWSER environment variable is set, evaluating the x-scheme-handler/http[s] before using BROWSER as fallback and explaining how to change the browser variable when it conflicts with whatever new setting one wants to use.

A followup would be updating the manpage.