qlang: Add parser for query filter language

With this parser, command line utilities will be able to provide filters
through query2 in a simplistic language. Example filters:

  name == "node3.example.com"
  master or (name == "node4.example.com")
  be/memory == 128 and name =~ /^web/i
  "inst1.example.com" in sinst_list
  status != "up"
  not master

Parts of the syntax came from Python, others from Perl. Documentation
will be added in follow-up patches.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>