redhat: do not install the chroot

SuSE doesn't allow it anymore. This is not needed since lldpd will do
it at start.
lib: silent a warning about uninitialized value

This is a false positive as the value was guarded by another
value. Nevertheless, let's please the compiler.
snmp: `agent_init()` won't modify AgentX string

This fixes another compiler warning.
xml: check that libxml-2 is really usable