collectd.conf: Fix default packet size of the network plugin.

Fixes: #710
EL <6 provide perl-ExtUtils-Embed with base perl

Conflicts:
	contrib/redhat/collectd.spec
java plugin: Make sure cjni_thread_detach() is called on all paths.

Each call to cjni_thread_attach() much be accompanied by
a call to cjni_thread_detach(). Some error handling cases were missing
the call, potentially locking the plugin up.
GenericJMXConfConnection: Remove unused imports.
RPM specfile: update changelog without incrementing release number
Merge branch 'collectd-5.3' into collectd-5.4

Conflicts:
	contrib/redhat/collectd.spec