remove nearly-duplicate PATHs in several places

function                                             old     new   delta
bb_PATH_root_path                                      -      35     +35
varunset                                              60      58      -2
arith                                               2042    2033      -9
bb_default_root_login_path                            30       -     -30
.rodata                                           128794  128762     -32
which_main                                           152     117     -35
defpathvar                                            40       -     -40
------------------------------------------------------------------------------
(add/remove: 1/2 grow/shrink: 0/4 up/down: 35/-148)          Total: -113 bytes

diff --git a/shell/msh.c b/shell/msh.c
index 2ee5256..dc2fa9c 100644
--- a/shell/msh.c
+++ b/shell/msh.c
@@ -5213,9 +5213,9 @@
 	if (path->value == null) {
 		/* Can be merged with same string elsewhere in bbox */
 		if (geteuid() == 0)
-			setval(path, bb_default_root_login_path);
+			setval(path, bb_default_root_path);
 		else
-			setval(path, bb_default_login_path);
+			setval(path, bb_default_path);
 	}
 	export(path);