add standard lua os methods, another fix for vfs links within links, allow write on urlfs mounted files

sys/init/3.sys.lua

@@ -1,3 +1,42 @@
-local fs = _G.fs
+local fs     = _G.fs
+local os     = _G.os
 
 fs.loadTab('sys/etc/fstab')
+
+-- add some Lua compatibility functions
+function os.remove(a)
+	if fs.exists(a) then
+        local s = pcall(fs.delete, a)
+        return s and true or nil, a .. ': Unable to remove file'
+	end
+	return nil, a .. ': No such file or directory'
+end
+
+os.execute = function(cmd)
+    if not cmd then
+        return 1
+    end
+
+    local env = _G.getfenv(2)
+    local s, m = env.shell.run('sys/apps/shell.lua ' .. cmd)
+
+    if not s then
+        return 1, m
+    end
+
+    return 0
+end
+
+os.tmpname = function()
+    local fname
+    repeat
+        fname = 'tmp/a' .. math.random(1, 32768)
+    until not fs.exists(fname)
+
+    return fname
+end
+
+-- non-standard - will raise error instead
+os.exit = function(code)
+    error('Terminated with ' .. code)
+end