patch 9.1.0899: default for 'backspace' can be set in C code

Problem:  default for 'backspace' can be set in C code
Solution: promote the default for 'backspace' from defaults.vim to the C
          code (Luca Saccarola)

closes: #16143

Signed-off-by: Luca Saccarola <github.e41mv@aleeas.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/optiondefs.h b/src/optiondefs.h
index ca085d4..2959232 100644
--- a/src/optiondefs.h
+++ b/src/optiondefs.h
@@ -414,9 +414,9 @@
 			    (char_u *)"light",
 #endif
 					    (char_u *)0L} SCTX_INIT},
-    {"backspace",   "bs",   P_STRING|P_VI_DEF|P_VIM|P_ONECOMMA|P_NODUP,
+    {"backspace",   "bs",   P_STRING|P_VIM|P_ONECOMMA|P_NODUP,
 			    (char_u *)&p_bs, PV_NONE, did_set_backspace, expand_set_backspace,
-			    {(char_u *)"", (char_u *)0L} SCTX_INIT},
+			    {(char_u *)"", (char_u *)"indent,eol,start"} SCTX_INIT},
     {"backup",	    "bk",   P_BOOL|P_VI_DEF|P_VIM,
 			    (char_u *)&p_bk, PV_NONE, NULL, NULL,
 			    {(char_u *)FALSE, (char_u *)0L} SCTX_INIT},