patch 8.0.1033: detecting background color does not work in screen

Problem:    Detecting background color does not work in screen, even when it
            is working like an xterm.
Solution:   Make "screen.xterm" use termcap entries like an xterm. (Lubomir
            Rintel, closes #2048)  When termresponse version is huge also
            recognize as not being an xterm.

src/os_unix.c

@@ -2280,6 +2280,7 @@ vim_is_xterm(char_u *name)
 		|| STRNICMP(name, "kterm", 5) == 0
 		|| STRNICMP(name, "mlterm", 6) == 0
 		|| STRNICMP(name, "rxvt", 4) == 0
+		|| STRNICMP(name, "screen.xterm", 12) == 0
 		|| STRCMP(name, "builtin_xterm") == 0);
 }