patch 9.0.0138: not enough characters accepted for 'spellfile'

Problem:    Not enough characters accepted for 'spellfile'.
Solution:   Add vim_is_fname_char() and use it for 'spellfile'.
diff --git a/src/spell.c b/src/spell.c
index 24abce4..8ca9313 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -4363,7 +4363,7 @@
     char_u *s;
 
     for (s = val; *s != NUL; ++s)
-	if (!vim_isfilec(*s) && *s != ',' && *s != ' ')
+	if (!vim_is_fname_char(*s))
 	    return FALSE;
     return TRUE;
 }