Automatic compaction selection on Linux (#2785)

* add auto compaction types

* pass db paths

* detect drive type on Linux

* use base db path

* add docstring

* limit the test to be side effect free

* use base db path

* more docs

* fix parsing test

* update error

* detect only on Linux

* make test Linux only

* add second device letter, update cli doc

* use spaces in cli doc

* import only on linux

* default->auto

util/src/lib.rs

@@ -104,6 +104,7 @@ extern crate parking_lot;
 extern crate ansi_term;
 extern crate tiny_keccak;
 extern crate rlp;
+extern crate regex;
 
 #[macro_use]
 extern crate heapsize;