Standardize `BROWSER` and `MAILER` envirnment variables.

Both the BROWSER and MAILER environment variables have no real standard behind them except for how xdg-utils and others implement them.

I'll cross post to the xdg mailing list once I'm no longer in the middle of creating a MR.

Quick Summary of the current behavior of xdg-utils:

BROWSER

Colon (:) separated list of programs, may have arguments and acts as a fallback URI handler.

Each command may have arguments (quoting is NOT supported).

The URI can be appended as the last argument or, when one %s is specified it is replaced with the URI instead of appending as the last argument.

When one program exits with an error it falls back to the next one. When called an browser that displays the file or URI is supposed to open.

MAILER

Colon (:) separated list of programs, may have arguments, a mailto: URI can be appended, when one program exits with an error it falls back to the next one. When called an interactive E-Mail editor, prepopulated from the URI is supposed to open.

Quoting shouldn't be relied upon.