patch 8.1.2169: terminal flags are never reset

Problem:    Terminal flags are never reset.
Solution:   Reset the flags when setting 'term'.
diff --git a/src/term.c b/src/term.c
index 2158fe3..b287121 100644
--- a/src/term.c
+++ b/src/term.c
@@ -1957,6 +1957,8 @@
 #if defined(UNIX) || defined(VMS)
     term_is_xterm = vim_is_xterm(term);
 #endif
+    is_not_xterm = FALSE;
+    is_mac_terminal = FALSE;
 
 #ifdef FEAT_MOUSE
 # if defined(UNIX) || defined(VMS)