Prevent use of shell as a hashbang program

This doesn't do what you want (it'll infinite loop), so while special
casing it /is/ ugly, it's better than a confusing error.

Closes #1386.

projects/core/src/main/resources/data/computercraft/lua/rom/programs/shell.lua

@@ -132,6 +132,13 @@ local function executeProgram(remainingRecursion, path, args)
         if not resolvedHashbangProgram then
             printError("Hashbang program not found: " .. originalHashbangPath)
             return false
+        elseif resolvedHashbangProgram == "rom/programs/shell.lua" and #hashbangArgs == 0 then
+            -- If we try to launch the shell then our shebang expands to "shell <program>", which just does a
+            -- shell.run("<program>") again, resulting in an infinite loop. This may still happen (if the user
+            -- has a custom shell), but this reduces the risk.
+            -- It's a little ugly special-casing this, but it's probably worth warning about.
+            printError("Cannot use the shell as a hashbang program")
+            return false
         end
 
         -- Add the path and any arguments to the interpreter's arguments

projects/core/src/test/resources/test-rom/spec/programs/shell_spec.lua

@@ -21,14 +21,16 @@ describe("The shell", function()
 
             expect(args):same { [0] = "/test-rom/data/dump-args", "arg1", "arg 2" }
         end)
+    end)
 
+    describe("hashbangs", function()
         local function make_hashbang_file(target, filename)
             local tmp = fs.open(filename or "test-files/out.lua", "w")
             tmp.write("#!" .. target)
             tmp.close()
         end
 
-        it("supports hashbangs", function()
+        it("has basic support", function()
             make_hashbang_file("/test-rom/data/dump-args")
             shell.execute("test-files/out.lua", "arg1", "arg2")
 
@@ -82,6 +84,16 @@ describe("The shell", function()
             make_hashbang_file("test-files/out.lua")
             expect(shell.execute("test-files/out.lua")):eq(false)
         end)
+
+        it("returns error for using the shell", function()
+            make_hashbang_file("shell")
+            expect(shell.execute("test-files/out.lua")):eq(false)
+        end)
+
+        it("allows running a shell with arguments", function()
+            make_hashbang_file("shell /test-rom/data/dump-args")
+            expect(shell.execute("test-files/out.lua")):eq(true)
+        end)
     end)
 
     describe("shell.run", function()