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/libbb/setup_environment.c b/libbb/setup_environment.c
index a98b9a5..a6f44f7 100644
--- a/libbb/setup_environment.c
+++ b/libbb/setup_environment.c
@@ -56,7 +56,7 @@
 		xsetenv("SHELL",   shell);
 		xsetenv("USER",    pw->pw_name);
 		xsetenv("LOGNAME", pw->pw_name);
-		xsetenv("PATH",   (pw->pw_uid ? bb_default_login_path : bb_default_root_login_path));
+		xsetenv("PATH",   (pw->pw_uid ? bb_default_path : bb_default_root_path));
 	}
 	else if (changeenv) {
 		/* Set HOME, SHELL, and if not becoming a super-user,