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

diff --git a/util-linux/mdev.c b/util-linux/mdev.c
index 34cabc9..956de15 100644
--- a/util-linux/mdev.c
+++ b/util-linux/mdev.c
@@ -276,8 +276,7 @@
 		putenv(s);
 		if (system(command) == -1)
 			bb_perror_msg_and_die("can't run '%s'", command);
-		s[4] = '\0';
-		unsetenv(s);
+		unsetenv("MDEV");
 		free(s);
 		free(command);
 	}