patch 9.0.2150: Using int for errbuflen in option funcs

Problem:  Using int for errbuflen in option funcs
Solution: Use size_t for errbuflen in string option functions

Use size_t for errbuflen in string option functions

This started as size_t to int warning on Windows in
do_set_option_string().  Other option setting functions are using size_t
for errbuflen so I changed the type in the argument list and then chased
further compiler warnings into the string option functions in
optionstr.c changing ints to size_t as I went.

I also changed the type of os_errbuflen in optset_T to size_t and
updated related argument lists in optionstr.c, as well as updating
set_string_option() to also use size_t for errbuflen for consistency
with other string option setting functions.

closes: #13628

Signed-off-by: Mike Williams <mikew@globalgraphics.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/option.c

@@ -1932,7 +1932,7 @@ do_set_option_string(
 	int	    cp_val,
 	char_u	    *varp_arg,
 	char	    *errbuf,
-	int	    errbuflen,
+	size_t	    errbuflen,
 	int	    *value_checked,
 	char	    **errmsg)
 {