Solaris/Illumos support

I've recently had to build node_exporter (from git master) for an Oracle Solaris system (11.3), and noticed that the 'Solaris' support is actually currently Illumos/OpenIndiana support. Both systems have different sets of ZFS kstat information available, and hence the zfs collector doesn't work on Oracle Solaris as-is.

I've modified the collector to properly support both flavours, and enabled the 'uname' collector (bsd variety) for Solaris/Illumos.

I've also added a filesystem collector for Solaris/Illumos based on the Linux filesystem collector, but using /etc/mnttab instead. I realise this strictly breaks the rules, namely 'A Collector may only read /proc or /sys files', however under Solaris the kernel mount table is mounted via the 'mnttab' FS as /etc/mnttab.

I also had to tweak Makefile.common to enable a build on both systems, as by default the build expects to be able to download a 'promu' binary, and solaris/illumos are not part of the standard releases.
This doesn't feel very complete to me, as the build will not fully work as expected on either platform without first manually building 'promu', and the vetting will still fail on both platforms due to an attempt to download promtool (no solaris prometheus binaries available). However it will at least now build on both platforms.

Code is at https://github.com/dsnt02518/node_exporter/tree/dsnt02518/solaris_illumos but I'm not sure this is ready for a PR yet?

Let me know if this should be broken out into multiple issues or if any more info is needed.

Start by reviewing the dsnt02518/solaris_illumos branch linked in the issue, focusing on the zfs and uname collectors, the proposed filesystem collector, and Makefile.common. Check how the changes support both Oracle Solaris and Illumos/OpenIndiana, then verify that both platforms build and that the stated promu and promtool limitations are addressed or documented.