Commit message:
```
Add CentOS packages and mark nss as patched
- add the following packages:
- libicu
- libunwind
- lttng-ust
- userspace-rcu (aka urcu)
- fix pkg-plist for CentOS 7:
- alsa-plugins-oss
- alsa-plugins-pusleaudio
- openssl
- nss:
- indicate that the versions of nss we ship have been patched [1][2]
[1] https://lists.centos.org/pipermail/centos-announce/2017-September/022552.html
[2] https://lists.centos.org/pipermail/centos-announce/2017-September/022550.html
```