libsystemd-bus: true/false instead of yes/no in msg dump

libsystemd-bus: true/false instead of yes/no in msg dump

Due to this patch, message dump (for message which includes boolean
type) is more consistent with dbus-send (which display true/false
instead of yes/no for boolean). It's only simple 'cosmetics change'.

** For dbus-send **

dbus-send --system --dest=org.freedesktop.DBus --type=method_call --print-reply / org.freedesktop.DBus.NameHasOwner string:org.freedesktop.login1

method return sender=org.freedesktop.DBus -> dest=:1.97 reply_serial=2
   boolean true

** For libsystemd-bus (without this patch) **

‣ Type=method_call  Endian=l  Flags=0  Version=2 Serial=8
  Destination=org.freedesktop.DBus  Path=/org/freedesktop/DBus  Interface=org.freedesktop.DBus  Member=NameHasOwner
  MESSAGE "s" {
  	STRING "org.freedesktop.login1";
  };

‣ Type=method_return  Endian=l  Flags=1  Version=2 Serial=51  ReplySerial=8
  Sender=:1.59  Destination=:1.67
  UniqueName=:1.59  WellKnownNames={org.freedesktop.DBus}
  MESSAGE "b" {
  	BOOLEAN yes;
  };

For me true/false seems to be better readable than yes/no for BOOLEAN.