Searcher: fixed default searcher selection

If none is specified, it is DFS.

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>
docs: explicitely select random searcher

DFS is enabled by default now.

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>
docs: clarified that concolic mode with DFS is the default setting

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>