utils/os: Added sdb_get_current_user().

… copied from sysdb/main.c which now uses this new function.
frontend: Get the username of the remote peer from a UNIX socket.

Then, only allow that user to authenticate against the daemon. That is, use
the same as PostgreSQL's "peer" authentication for all connections.
Merged branch 'master' of git://git.tokkee.org/sysdb.
utils/proto: Replaced sdb_proto_send_msg with sdb_proto_marshal.

The new function won't actually send the message but write it into a provided
buffer in the wire format. This allows for more flexible use of the function.
connection: Renamed sdb_connection_read to sdb_connection_handle.

The function does more than read from the connection, so the old name was a
bad choice.
connection: Let sdb_connection_close not deallocate the connection.

That'll allow for more flexible use.