frontend parser: Added support for parsing single expressions.

The parser has been extended to support different modes. In SDB_PARSE_EXPR
mode, a single expression will be accepted as a start condition. The new
function sdb_fe_parse_matcher() uses this to parse expressions.

For now, this implements only the basic framework. Any identifier will be
parsed as a simple name matcher expression.
utils llist: Let llist_get() increment the reference count.

This feels more consistent.
frontend: Let the parser not accept other grammars in default mode.
object: Let create_simple() accept an optional destructor.

This is still simple but it allows for some more dynamic.
object: Added sdb_object_create_dT() macro.

This may be used to create a simple dynamic object of a specified type.
Dynamic type means that it uses a destructor.
frontend: Added 'FETCH <name>' query command.

For now, <name> may only be a simple identifier.
parser_test: Check the node's command type.
frontend: Added FETCH support to the connection handler.

The wire format is: <CONNECTION_FETCH>;<len>;<host>
frontend: Free dynamically allocated memory in the parser.
frontend: Added very simple single-quoted string support.

No escape sequences whatsoever are supported so far but except for that any
single-quoted strings which do not contain single quotes, can be used.

FETCH now supports host names specified as strings as well.
frontend: Require strings instead of identifiers.

… for host names and name matchers. Identifiers were used as an early
work-around but they are not valid in those places.