run-mailcap: accept special URL type to open with sensible-browser, give URL-like (^[a-z_-]+://) nonexistent arguments the URL type

With the open alternative being run-mailcap, it'd still be very nice to be able to open http://whatever.

(On my laptop I additionally have a local diversion and alternative that provides xdg-open via run-mailcap, because some programs explicitly run xdg-open and the override mechanism for that overwhelmed me when all I wanted to do is application/pdf; firefox %s, which is how I got here. I think this is a relatively sane desire, even if my setup isn't, since /usr/bin/open is in alternatives by default.)

Closes: #291015. Notionally this solves a different problem, inasmuch as the OP wanted run-mailcap to also handle some schemes specially. That was in 2005, and I think the intervening 18 years show that that's weird and/or confusing – I haven't seen a URL with an authority section (://) that wasn't meant to be opened in a browser and where it made sense to strip the scheme and somehow guess what else to do at the level where you're running open. The exception may be file://, but, again, confusing – I expect file:///whatever to open file:///whatever in firefox, not pre-process it in some way to potentially open it in some other way.

First commit is a no-op that just moves stuff (hopefully; I'm not a real Perl enjoyer, but it works for me). Second adds the thing.