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/messages.c b/libbb/messages.c
index 27d2cc1..56cccaf 100644
--- a/libbb/messages.c
+++ b/libbb/messages.c
@@ -40,7 +40,7 @@
 const char bb_default_login_shell[] = LIBBB_DEFAULT_LOGIN_SHELL;
 /* util-linux manpage says /sbin:/bin:/usr/sbin:/usr/bin,
  * but I want to save a few bytes here. Check libbb.h before changing! */
-const char bb_default_root_login_path[] = "/sbin:/usr/sbin:/bin:/usr/bin";
+const char bb_PATH_root_path[] = "PATH=/sbin:/usr/sbin:/bin:/usr/bin";
 
 
 const int const_int_0;