fix: offer a drive by the folder its system reads

RetroArch's folder question numbered a Batocera SHARE drive and filled
its root. A drive is now offered by its own system's BIOS folder, one
nothing names by its bios folder, and another system's is left out and
named with the options that reach it. A list or pack card setups read
too opens on those setups, in both modes.

wiki/installer.md

@@ -1031,9 +1031,16 @@ and `--plain` gives the same list numbered, in the same order, with `--dest`
 too. Both find the SD cards mounted here, the screen under **Found on this
 machine**. With a setup found, the setup screen lists it with **Another setup
 or folder**, and `--plain` asks the same: Enter takes the first, `o` opens the
-numbered list and then the folder, `q` quits. A platform named, or read from
-a list or a pack, and found nowhere gets its folder asked, its usual one under
-Enter, the mounted drives numbered. A folder typed is read from the current
+numbered list and then the folder, `q` quits. A platform named and found
+nowhere gets its folder asked, its usual one under Enter, the mounted drives
+numbered. A list or a pack read by card setups too (RetroArch's, read by Onion,
+muOS, ArkOS and the others) opens the setup list instead, filtered to the
+setups that read it, RetroArch first: Enter on it asks RetroArch's folder. At
+that question a drive is numbered by the folder its system reads (`ONION:
+BIOS`), a drive nothing names by its `bios` folder (`EASYROMS: bios`), never
+by its root, which no setup reads. Another system's card or SHARE drive, and a
+drive two systems fit, are left out, and a line names them: each is reached
+through its own setup. A folder typed is read from the current
 one when it is not a full path, and named in full after. `q` quits there too
 (a folder named `q` is typed `./q`), and Enter with no default says what to
 type. On a computer Batocera and Recalbox have no default: the drives are