systemctl: for switch-root check, if we switch to a systemd init

If "systemctl switch-root" is called with a specific "INIT" or
/proc/cmdline contains "init=", then systemd would not serialize
itsself.

Let systemctl check, if the new init is in the standard systemd
installation path and if so, clear the INIT parameter,
to let systemd serialize itsself.

src/shared/util.h

@@ -813,6 +813,19 @@ int unlink_noerrno(const char *path);
                 _c_;                                    \
         })
 
+#define strappenda3(a, b, c)                                    \
+        ({                                                      \
+                const char *_a_ = (a), *_b_ = (b), *_c_ = (c);  \
+                char *_d_;                                      \
+                size_t _x_, _y_, _z_;                           \
+                _x_ = strlen(_a_);                              \
+                _y_ = strlen(_b_);                              \
+                _z_ = strlen(_c_);                              \
+                _d_ = alloca(_x_ + _y_ + _z_ + 1);              \
+                strcpy(stpcpy(stpcpy(_d_, _a_), _b_), _c_);     \
+                _d_;                                            \
+        })
+
 #define procfs_file_alloca(pid, field)                                  \
         ({                                                              \
                 pid_t _pid_ = (pid);                                    \