patch 8.1.1966: some code in options.c fits better elsewhere

Problem:    Some code in options.c fits better elsewhere.
Solution:   Move functions from options.c to other files. (Yegappan
            Lakshmanan, closes #4889)
diff --git a/src/globals.h b/src/globals.h
index 1cc7b9f..46dedae 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1550,6 +1550,7 @@
 EXTERN char e_cannot_mod[]	INIT(= N_("E995: Cannot modify existing variable"));
 EXTERN char e_readonlyvar[]	INIT(= N_("E46: Cannot change read-only variable \"%s\""));
 EXTERN char e_readonlysbx[]	INIT(= N_("E794: Cannot set variable in the sandbox: \"%s\""));
+EXTERN char e_stringreq[]	INIT(= N_("E928: String required"));
 EXTERN char e_emptykey[]	INIT(= N_("E713: Cannot use empty key for Dictionary"));
 EXTERN char e_dictreq[]	INIT(= N_("E715: Dictionary required"));
 EXTERN char e_listidx[]	INIT(= N_("E684: list index out of range: %ld"));