misc: Use gmtime for conversion to UTC to avoid getenv/setenv

This patch changes how we turn time into UTC. Previously we
manipulated the TZ environment variable, but this has issues as the
strings that are manipulated could be tainted (see e.g. CERT
ENV34-C). Now we simply rely on the built-in gmtime function and avoid
touching getenv/setenv all together.

src/base/time.cc

@@ -150,18 +150,7 @@ sleep(const Time &time)
 time_t
 mkutctime(struct tm *time)
 {
-    time_t ret;
-    char *tz;
-
-    tz = getenv("TZ");
-    setenv("TZ", "", 1);
-    tzset();
-    ret = mktime(time);
-    if (tz)
-        setenv("TZ", tz, 1);
-    else
-        unsetenv("TZ");
-    tzset();
-    return ret;
+    time_t local = mktime(time);
+    return mktime(gmtime(&local));
 }
 

src/dev/sparc/dtod.cc

@@ -53,17 +53,8 @@ DumbTOD::DumbTOD(const Params *p)
     : BasicPioDevice(p, 0x08)
 {
     struct tm tm = p->time;
-    char *tz;
-
-    tz = getenv("TZ");
-    setenv("TZ", "", 1);
-    tzset();
-    todTime = mktime(&tm);
-    if (tz)
-        setenv("TZ", tz, 1);
-    else
-        unsetenv("TZ");
-    tzset();
+    time_t local = mktime(&tm);
+    todTime = mktime(gmtime(&local));
 
     DPRINTFN("Real-time clock set to %s\n", asctime(&tm));
     DPRINTFN("Real-time clock set to %d\n", todTime);