patch 8.0.0713: 'termkey' option not fully implemented

Problem:    'termkey' option not fully implemented.
Solution:   Add initialisation.
diff --git a/src/option.c b/src/option.c
index 82b34dc..39aef6e 100644
--- a/src/option.c
+++ b/src/option.c
@@ -10901,6 +10901,7 @@
     to->wo_cole = from->wo_cole;
 #endif
 #ifdef FEAT_TERMINAL
+    to->wo_tk = vim_strsave(from->wo_tk);
     to->wo_tms = vim_strsave(from->wo_tms);
 #endif
 #ifdef FEAT_FOLDING
@@ -10970,6 +10971,7 @@
     check_string_option(&wop->wo_cocu);
 #endif
 #ifdef FEAT_TERMINAL
+    check_string_option(&wop->wo_tk);
     check_string_option(&wop->wo_tms);
 #endif
 #ifdef FEAT_LINEBREAK
@@ -11012,6 +11014,7 @@
     clear_string_option(&wop->wo_cocu);
 #endif
 #ifdef FEAT_TERMINAL
+    clear_string_option(&wop->wo_tk);
     clear_string_option(&wop->wo_tms);
 #endif
 }