libbb: add bb_unsetenv (taken from hush).

udhcpc: stop filtering environment passed to the script.
crond: fix uncovered potential bug (failing unsetenv)
mdev: fix uncovered potential bug (failing unsetenv)
tcp, udpsvd: fix uncovered potential bug (failing unsetenv)

function                                             old     new   delta
safe_setenv                                            -      58     +58
bb_unsetenv                                            -      55     +55
builtin_unset                                        139     138      -1
tcpudpsvd_main                                      1843    1830     -13
free_strings_and_unsetenv                             87      53     -34
udhcp_run_script                                    1186    1133     -53
safe_setenv4                                          62       -     -62
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 0/4 up/down: 113/-163)          Total: -50 bytes

1 files changed, 3 insertions, 11 deletions

diff --git a/shell/hush.c b/shell/hush.c
index eafcbb4c9..3b87855b6 100644
--- a/shell/hush.c
+++ b/shell/hush.c
@@ -730,16 +730,8 @@ static void free_strings_and_unsetenv(char **strings, int unset)
         v = strings;
         while (*v) {
                 if (unset) {
-                        char *copy;
-                        /* *strchrnul(*v, '=') = '\0'; -- BAD
-                         * In case *v was putenv'ed, we can't
-                         * unsetenv(*v) after taking out '=':
-                         * it won't work, env is modified by taking out!
-                         * horror :( */
-                        copy = xstrndup(*v, strchrnul(*v, '=') - *v);
-                        debug_printf_env("unsetenv '%s'\n", copy);
-                        unsetenv(copy);
-                        free(copy);
+                        debug_printf_env("unsetenv '%s'\n", *v);
+                        bb_unsetenv(*v);
                 }
                 free(*v++);
         }
@@ -2937,7 +2929,7 @@ static void unset_local_var(const char *name)
                          * is ro, and we cannot reach this code on the 1st pass */
                         prev->next = cur->next;
                         debug_printf_env("%s: unsetenv '%s'\n", __func__, cur->varstr);
-                        unsetenv(cur->varstr);
+                        bb_unsetenv(cur->varstr);
                         if (!cur->max_len)
                                 free(cur->varstr);
                         free(cur);