amqp plugin: Added support for rabbitmq-c 0.4.x.

Upstream introduced a new socket interface and deprecated the old one. This
leads to compiler errors when using GCC and -Werror.
amqp plugin: Fixed compilation when using rabbitmq-c < 0.4.

Thanks to Marc Fournier for noticing!
fix cgroups plugin name in example config
[network] set_thread_cbs so we initialize the right threading mode in gcry_check_version

Signed-off-by: Florian Forster <octo@collectd.org>
apache plugin: Call curl_global_init() from the init function.

This is a shot in the dark in trying to address #513. By calling this
from an init() callback, I hope to be initializing the curl and gcrypt
libraries before collectd becomes multi-threaded, avoiding the problems
described in the issue.
network: comment libgcrypt initalization process
amqp: fix symbol lookup error with librabbitmq >= 0.4.0

amqp_socket_close() which we were using isn't exported anymore.
See https://github.com/alanxz/rabbitmq-c/commit/bdda7ab

amqp_destroy_connection() implicitly closes the socket, so we don't need
to bother about this if using librabbitmq >= 0.4.0

credits to @tokkee for figuring that out !
Merge pull request #2 from mfournier/amqp-symbol_lookup

amqp: fix symbol lookup error with librabbitmq >= 0.4.0
amqp plugin: Don't use C++ style comments.
configure: Fixed linker options when checking for amqp_tcp_socket_new.

This fixes the check when using GCC's --as-needed which fails when options are
not specified in the right order. Libraries (-l options) have to be specified
after any source files. autoconf does the right thing when using $LIBS for
that case rather than $LDFLAGS.
Merge remote-tracking branch 'origin/pr/490' into collectd-5.3
Merge remote-tracking branch 'origin/collectd-4.10' into collectd-5.3
Merge branch 'collectd-5.3' into collectd-5.4
Bump version to 5.4.1; Update ChangeLog.
Please free strings *after* printing them ;-)

Signed-off-by: Florian Forster <octo@collectd.org>
configure: Fixed check for libsigrok and AM_PATH_GLIB_2_0.
Collect drbd statistics on linux
lvm: Do not segfault when there are no VGs
write_riemann plugin: Receive acknowledge message when using TCP.

Not receiving an acknowledge message when communicating with riemann over TCP
will cause the riemann instance to eventually hang for extended periods of time
because of resource exhaustion.

Took the time to reaorganize the riemann_send function to simplify locking.
link iptables plugin against flags from pkg-config

This solves issue #326 for ubuntu versions >= 13.10