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, 23 insertions, 0 deletions

diff --git a/libbb/xfuncs_printf.c b/libbb/xfuncs_printf.c
index 108e14043..46ae7ac60 100644
--- a/libbb/xfuncs_printf.c
+++ b/libbb/xfuncs_printf.c
@@ -333,6 +333,29 @@ void FAST_FUNC xsetenv(const char *key, const char *value)
                 bb_error_msg_and_die(bb_msg_memory_exhausted);
 }
 
+/* Handles "VAR=VAL" strings, even those which are part of environ
+ * _right now_
+ */
+void FAST_FUNC bb_unsetenv(const char *var)
+{
+        char *tp = strchr(var, '=');
+
+        if (!tp) {
+                unsetenv(var);
+                return;
+        }
+
+        /* In case var was putenv'ed, we can't replace '='
+         * with NUL and unsetenv(var) - it won't work,
+         * env is modified by the replacement, unsetenv
+         * sees "VAR" instead of "VAR=VAL" and does not remove it!
+         * horror :( */
+        tp = xstrndup(var, tp - var);
+        unsetenv(tp);
+        free(tp);
+}
+
+
 // Die with an error message if we can't set gid.  (Because resource limits may
 // limit this user to a given number of processes, and if that fills up the
 // setgid() will fail and we'll _still_be_root_, which is bad.)