Add setting bios.shell_path

Allow the user to directly set the path the bios uses for the shell.

Has some additional status checking to allow for shell errors to be read before computer shutdown.

projects/core/src/main/resources/data/computercraft/lua/bios.lua

@@ -697,6 +697,20 @@ if term.isColour() then
         type = "boolean",
     })
 end
+
+local sShell
+if term.isColour() and settings.get("bios.use_multishell") then
+    sShell = "rom/programs/advanced/multishell.lua"
+else
+    sShell = "rom/programs/shell.lua"
+end
+
+settings.define("bios.shell_path", {
+    default = sShell,
+    description = "The path the bios executes as the shell. This program is responsible for implementing the shell and multishell API, handling user input, and program execution.",
+    type = "string"
+})
+
 if _CC_DEFAULT_SETTINGS then
     for sPair in string.gmatch(_CC_DEFAULT_SETTINGS, "[^,]+") do
         local sName, sValue = string.match(sPair, "([^=]*)=(.*)")
@@ -728,24 +742,24 @@ if fs.exists(".settings") then
 end
 
 -- Run the shell
+local shellOk
 local ok, err = pcall(parallel.waitForAny,
     function()
-        local sShell
-        if term.isColour() and settings.get("bios.use_multishell") then
-            sShell = "rom/programs/advanced/multishell.lua"
-        else
-            sShell = "rom/programs/shell.lua"
+        shellOk = os.run({}, settings.get("bios.shell_path"))
+        if shellOk then
+            os.run({}, "rom/programs/shutdown.lua")
         end
-        os.run({}, sShell)
-        os.run({}, "rom/programs/shutdown.lua")
     end,
     rednet.run
 )
 
 -- If the shell errored, let the user read it.
 term.redirect(term.native())
-if not ok then
-    printError(err)
+if not (ok and shellOk) then
+    -- if the shell within os.run errored, then the error was already output, and the shell loop exited normally.
+    if not ok then
+        printError(err)
+    end
     pcall(function()
         term.setCursorBlink(false)
         print("Press any key to continue")