Ensure tsdb can be built on SmartOS/Illumos/Solaris
Ensure tsdb can be built on SmartOS/Illumos/Solaris

At the moment tsdb cannot be built on Illumos/Solaris due to

- An exclude tag against Solaris is added in db_unix.go
- We use built-in syscall package, which doesn't have nmmap and munmap support

This PR supports build on Illumos/Solaris by remove the Solaris exclude tag in db_unix.go
At the same time use golang.org/x/sys/unix package instead of syscall package, since it has Solaris nmmap and munmap implementation.

prometheus/prometheus,kangwoo/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,alileza/prometheus,Gouthamve/tsdb,Gouthamve/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,prometheus/tsdb,kangwoo/prometheus,alileza/prometheus,prometheus/prometheus,Gouthamve/prometheus,prometheus/prometheus,kangwoo/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,kangwoo/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus