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
curl_json plugin: separate the cj_curl_perform in to the curl and yajl parts
curl_json plugin: support getting json from a Unix socket as well as a URL
curl_json plugin: use sstrerr for thread safety
curl_json plugin: style cleanups
Merge branch 'pr/392'
check if libpcap has PCAP_ERROR_IFACE_NOT_UP

This should prevent DNS plugin from being built on systems with libpcap
older than 1.0.0.

GitHub issue #373
RedHat RPM spec: updates for 5.4.0
curl_json plugin: avoid unlikely overrun

Signed-off-by: Florian Forster <octo@collectd.org>
curl_json plugin: avoid accessing off the end of the avl_tree_s

It's not written this way, but really we have a union { *key; *tree; }
which is differentiated by checking for the presence a magic field
which only exists in key.  This leads to accesses off the end of the
tree.  Putting the magic at start of the key avoids this.

Signed-off-by: Florian Forster <octo@collectd.org>
zfs_arc plugin: Collect "allocated" and "stolen" on FreeBSD only.

The appropriate log messages have been demoted to WARNING.

Github: #305
cgroups: don't fail parsing when colon not found

The format of cpuacct.stat seems to not always have a colon as separator.

It appears to *not* have the colon at least on debian's kernel 3.2.41 and
3.2.35, as well as on RHEL6 with 2.6.32. All of them have decent support for
cgroups/lxc, and the cgroups plugin fails to work with them without this patch.

Also, looking at other implementations shows that they *don't* expect a colon:
https://bitbucket.org/dotcloud/liblxcstats/src/2558b4fbbf589c609895b0badbfc7d413466d716/probes/cpuacct.c?at=default#cl-71
https://github.com/BrightcoveOS/Diamond/blob/master/src/collectors/cpuacct_cgroup/cpuacct_cgroup.py#L55
correct typo
src/utils_mount.c: Fix wrong behavior in cu_mount_getoptionvalue().

Github: #412
cgroups plugin: Use cu_mount_checkoption().

cu_mount_checkoption() behaves a bit like strstr() in that it returns a pointer
into the buffer. cu_mount_getoptionvalue() allocated memory that was never
freed, slowly leaking memory (although very little).

Github: #412
Merge remote-tracking branch 'github/pr/413' into collectd-5.4
write_riemann: Add extra meta strings as attributes in notifications
correct default write_graphite protocol in manpage
Merge remote-tracking branch 'origin/pr/409' into collectd-5.4
collectd.conf: Added example cgroups plugin config section.