pid1: restore the original environment passed by the kernel when switching to a new system manager

PID1 may modified the environment passed by the kernel when it starts
running. Commit 9d48671c62 unset $HOME for
example.

In case PID1 is going to switch to a new root and execute a new system manager
which is not systemd, we should restore the original environment as the new
manager might expect some variables to be set by default (more specifically
$HOME).

src/basic/util.c

@@ -39,6 +39,7 @@
 #include "set.h"
 #include "signal-util.h"
 #include "stat-util.h"
+#include "static-destruct.h"
 #include "string-util.h"
 #include "strv.h"
 #include "time-util.h"
@@ -49,8 +50,11 @@
 
 int saved_argc = 0;
 char **saved_argv = NULL;
+char **saved_env = NULL;
 static int saved_in_initrd = -1;
 
+STATIC_DESTRUCTOR_REGISTER(saved_env, strv_freep);
+
 bool kexec_loaded(void) {
        _cleanup_free_ char *s = NULL;
 
@@ -298,3 +302,7 @@ void disable_coredumps(void) {
         if (r < 0)
                 log_debug_errno(r, "Failed to turn off coredumps, ignoring: %m");
 }
+
+void save_env(void) {
+        saved_env = strv_copy(environ);
+}