Optional message to user when terminating/cancelling backend

When terminating, or cancelling, a backend it’s currently not possible to let
the signalled session know *why* it was dropped.  This has nagged me in the
past and now it happened to come up again, so I took a stab at this.  The
attached patch implements the ability to pass an optional text message to the
signalled session which is included in the error message:

  SELECT pg_terminate_backend(<pid> [, message]);
  SELECT pg_cancel_backend(<pid> [, message]);

Right now the message is simply appended on the error message, not sure if
errdetail or errhint would be better? Calling:

  select pg_terminate_backend(<pid>, 'server rebooting');

..leads to:

  FATAL:  terminating connection due to administrator command: "server rebooting"

Omitting the message invokes the command just like today.

The message is stored in a new shmem area which is checked when the session is
aborted.  To keep things simple a small buffer is kept per backend for the
message.  If deemed too costly, keeping a central buffer from which slabs are
allocated can be done (but seemed rather complicated for little gain compared
to the quite moderate memory spend.) 

cheers ./daniel