Add GetHostOs to redbean.

tool/net/help.txt

@@ -639,6 +639,9 @@ FUNCTIONS
            Returns host associated with request. This will be the Host
            header, if it's supplied. Otherwise it's the bind address.
 
+   GetHostOs() → str
+           Returns string that describes the host OS.
+
    GetMonospaceWidth(str|char) → int
            Returns monospace display width of string. This is useful for
            fixed-width formatting. For example, CJK characters typically take

tool/net/redbean.c

@@ -5035,6 +5035,32 @@ static int LuaGetComment(lua_State *L) {
   return 1;
 }
 
+
+static int LuaGetHostOs(lua_State *L) {
+  const char *s = NULL;
+  if (IsLinux()) {
+    s = "LINUX";
+  } else if (IsMetal()) {
+    s = "METAL";
+  } else if (IsWindows()) {
+    s = "WINDOWS";
+  } else if (IsXnu()) {
+    s = "XNU";
+  } else if (IsOpenbsd()) {
+    s = "OPENBSD";
+  } else if (IsFreebsd()) {
+    s = "FREEBSD";
+  } else if (IsNetbsd()) {
+    s = "NETBSD";
+  }
+  if (s) {
+    lua_pushstring(L, s);
+  } else {
+    lua_pushnil(L);
+  }
+  return 1;
+}
+
 static void LuaSetIntField(lua_State *L, const char *k, lua_Integer v) {
   lua_pushinteger(L, v);
   lua_setfield(L, -2, k);
@@ -5237,6 +5263,7 @@ static const luaL_Reg kLuaFuncs[] = {
     {"GetHeader", LuaGetHeader},                                //
     {"GetHeaders", LuaGetHeaders},                              //
     {"GetHost", LuaGetHost},                                    //
+    {"GetHostOs", LuaGetHostOs},                                //
     {"GetHttpReason", LuaGetHttpReason},                        //
     {"GetHttpVersion", LuaGetHttpVersion},                      //
     {"GetLastModifiedTime", LuaGetLastModifiedTime},            //