*: introduce and use bb_unsetenv_and_free

function                                             old     new   delta
bb_unsetenv_and_free                                   -      17     +17
tcpudpsvd_main                                      1819    1810      -9
safe_setenv                                           58      47     -11
udhcp_run_script                                     630     616     -14
make_device                                         1683    1663     -20
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/4 up/down: 17/-54)            Total: -37 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/util-linux/mdev.c b/util-linux/mdev.c
index 2170756..b4042c0 100644
--- a/util-linux/mdev.c
+++ b/util-linux/mdev.c
@@ -374,10 +374,8 @@
 				putenv(s1);
 				if (system(command) == -1)
 					bb_perror_msg("can't run '%s'", command);
-				unsetenv("SUBSYSTEM");
-				free(s1);
-				unsetenv("MDEV");
-				free(s);
+				bb_unsetenv_and_free(s1);
+				bb_unsetenv_and_free(s);
 				free(command);
 			}