add log_logstash plugin to emit logstash json_event messages.
Add SNMPv3 authentication and encryption support

Adds the following configuration options to support SNMPv3 authentication:
* SecurityLevel - authPriv | authNoPriv | noAuthNoPriv
* Username - username for authentication
* AuthPassphrase - passphrase for authentication
* AuthProtocol - MD5 or SHA
* PrivacyPassphrase - passphrase for encryption
* PrivacyProtocol - AES or DES
* Context - Optional authentication context
Bump version to 5.3.1; Update ChangeLog.
lpar: make dots in configure.in consistent with other plugins
Initial libmnl porting attempt
Update configure script to link with -lmnl
Fix netlink.c to (almost) build
Fix HAVE_TCA_STATS(2) checks in configure.in
Restructure to compile with all combos of HAVE_TCA_STATS(2)

We want to compile with all combinations of
HAVE_TCA_STATS(2) set/unset and at the same time
protect against submitting the stats twice (if both
are supported).
Introduce "stats_found" and set and attribute on it
to avoid having the compiler complain about it.
This allows us to bury the specific structs under
each HAVE_TCA_STATS(2) #if ...
More fixing of netlink build with different HAVE_...
libmnl: fix 2 mistakes preventing building with --enable-debug
netlink: fix segfault & make advanced options work again

A type mismatch and offset mistake was preventing the interface list
from getting iterated. This was making collectd segfault when the
"QDisc", "Class" or "Filter" options were enabled.
netlink: indentation & whitespace/tab consistency
netlink: comments & cleanup
netlink: __attribute__ usage clarification

As requested by @octo in GitHub#376.
update README to mention libmnl dependency
update build system to reflect switch to libmnl

"./configure" was previously checking for libnetlink and setting
"-lmnl" if found, which was obviously calling for trouble. With
this patch, the last references to libnetlink are dead and buried.

NB: linux/netlink.h and friends are still included here and there
though, but these are not related to libnetlink.
rrdcached, rrdtool plugins: Honor the "DataDir" setting.

This fixes a regression introduced in:
bfd3f06ef4996707036756371c5724ebf2748821

Github: #380
rrdcached, rrdtool plugins: Add directory separator after datadir.

D'oh! Thanks to @rcpoison for pointing this out!

Github: #380
Fix pointer confusion for nested attribute parsing

A reference to a pointer is passed as data,
the resulting attribute payload should be stored in
the dereferenced location to return it to the caller.