dbus: limit the number of env variables to something reasonable, vol. 2

Turns out we can utilize this limit at a couple more places, so let's
move the previously defined limit constant to env-util.h and use it to
guard a couple more D-Bus methods. Also, bump it a bit, given it's meant
to be a safety cap that can't be hit in valid scenarios.

Follow-up for 7671b43cb8.

src/basic/env-util.h

@@ -3,6 +3,8 @@
 
 #include "basic-forward.h"
 
+#define ENVIRONMENT_ASSIGNMENTS_MAX 16384U
+
 size_t sc_arg_max(void);
 
 bool env_name_is_valid(const char *e);