*: 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/include/libbb.h b/include/libbb.h
index 5e962fd..58719a8 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -408,6 +408,7 @@
 void xchroot(const char *path) FAST_FUNC;
 void xsetenv(const char *key, const char *value) FAST_FUNC;
 void bb_unsetenv(const char *key) FAST_FUNC;
+void bb_unsetenv_and_free(char *key) FAST_FUNC;
 void xunlink(const char *pathname) FAST_FUNC;
 void xstat(const char *pathname, struct stat *buf) FAST_FUNC;
 int xopen(const char *pathname, int flags) FAST_FUNC;