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, 1 insertions, 2 deletions

diff --git a/networking/tcpudp.c b/networking/tcpudp.c
index 3b73f213f..55a3e0899 100644
--- a/networking/tcpudp.c
+++ b/networking/tcpudp.c
@@ -85,8 +85,7 @@ static void undo_xsetenv(void)
         char **pp = env_cur = &env_var[0];
         while (*pp) {
                 char *var = *pp;
-                *strchrnul(var, '=') = '\0';
-                unsetenv(var);
+                bb_unsetenv(var);
                 free(var);
                 *pp++ = NULL;
         }