patch 8.2.0590: no 'backspace' value allows ignoring the insertion point

Problem:    No 'backspace' value allows ignoring the insertion point.
Solution:   Add the "nostop" and 3 values. (Christian Brabandt, closes #5940)
diff --git a/src/optionstr.c b/src/optionstr.c
index 1cf1ee9..17613a6 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -68,7 +68,7 @@
 static char *(p_ead_values[]) = {"both", "ver", "hor", NULL};
 static char *(p_buftype_values[]) = {"nofile", "nowrite", "quickfix", "help", "terminal", "acwrite", "prompt", "popup", NULL};
 static char *(p_bufhidden_values[]) = {"hide", "unload", "delete", "wipe", NULL};
-static char *(p_bs_values[]) = {"indent", "eol", "start", NULL};
+static char *(p_bs_values[]) = {"indent", "eol", "start", "nostop", NULL};
 #ifdef FEAT_FOLDING
 static char *(p_fdm_values[]) = {"manual", "expr", "marker", "indent", "syntax",
 # ifdef FEAT_DIFF
@@ -1910,7 +1910,7 @@
     {
 	if (VIM_ISDIGIT(*p_bs))
 	{
-	    if (*p_bs > '2' || p_bs[1] != NUL)
+	    if (*p_bs > '3' || p_bs[1] != NUL)
 		errmsg = e_invarg;
 	}
 	else if (check_opt_strings(p_bs, p_bs_values, TRUE) != OK)