web--browse: look at the BROWSER env var

web--browse: look at the BROWSER env var

The BROWSER environment variables is a widely-accepted standard way to
set the user-preferred browser(s). It contains a colon-separate list of
commands to (try to) execute to open a web page. Each item in the list
is allowed to have a %s placeholder to be replaced by the URL, in which
case we try to run the command as is. If no placeholder is found, we
only look at the command name to see if it matches one of our known
browsers.