patch 9.0.1485: no functions for converting from/to UTF-16 index

Problem:    no functions for converting from/to UTF-16 index.
Solution:   Add UTF-16 flag to existing funtions and add strutf16len() and
            utf16idx(). (Yegappan Lakshmanan, closes #12216)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 5c10f1e..10d00d5 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -1751,9 +1751,9 @@
 			ret_number,	    f_bufwinnr},
     {"byte2line",	1, 1, FEARG_1,	    arg1_number,
 			ret_number,	    f_byte2line},
-    {"byteidx",		2, 2, FEARG_1,	    arg2_string_number,
+    {"byteidx",		2, 3, FEARG_1,	    arg3_string_number_bool,
 			ret_number,	    f_byteidx},
-    {"byteidxcomp",	2, 2, FEARG_1,	    arg2_string_number,
+    {"byteidxcomp",	2, 3, FEARG_1,	    arg3_string_number_bool,
 			ret_number,	    f_byteidxcomp},
     {"call",		2, 3, FEARG_1,	    arg3_any_list_dict,
 			ret_any,	    f_call},
@@ -1803,7 +1803,7 @@
 			ret_number,	    f_charclass},
     {"charcol",		1, 2, FEARG_1,	    arg2_string_or_list_number,
 			ret_number,	    f_charcol},
-    {"charidx",		2, 3, FEARG_1,	    arg3_string_number_bool,
+    {"charidx",		2, 4, FEARG_1,	    arg3_string_number_bool,
 			ret_number,	    f_charidx},
     {"chdir",		1, 1, FEARG_1,	    arg1_string,
 			ret_string,	    f_chdir},
@@ -2601,6 +2601,8 @@
 			ret_number,	    f_strridx},
     {"strtrans",	1, 1, FEARG_1,	    arg1_string,
 			ret_string,	    f_strtrans},
+    {"strutf16len",	1, 2, FEARG_1,	    arg2_string_bool,
+			ret_number,	    f_strutf16len},
     {"strwidth",	1, 1, FEARG_1,	    arg1_string,
 			ret_number,	    f_strwidth},
     {"submatch",	1, 2, FEARG_1,	    arg2_number_bool,
@@ -2785,6 +2787,8 @@
 			ret_dict_any,	    f_undotree},
     {"uniq",		1, 3, FEARG_1,	    arg13_sortuniq,
 			ret_first_arg,	    f_uniq},
+    {"utf16idx",	2, 4, FEARG_1,	    arg3_string_number_bool,
+			ret_number,	    f_utf16idx},
     {"values",		1, 1, FEARG_1,	    arg1_dict_any,
 			ret_list_member,    f_values},
     {"virtcol",		1, 2, FEARG_1,	    arg2_string_or_list_bool,