Basic subtransaction facility

Hackers,

Here is a very preliminar patch that allows the user to say "BEGIN"
inside a transaction and have the system react accordingly. This is
only a modification to xact.c (and slightly to other places to allow it
to work); the important functions are empty.

It compiles fine for me with both SUBTRANSACTIONS defined and not
defined; when not defined, the behavior is the same as the current code.
Please note that I have made some errors more fatal than they are now,
as bugs in this code will have much worse effects than a flaw in the
current transaction system.

One quick note: there are two ENDABORT states for a subtransaction,
SUBENDABORT_OK and SUBENDABORT_ERROR. They signal whether the parent
transaction should be aborted after the child transaction finishes or
not: an aborted subtransaction where the user issues COMMIT should
abort the parent transaction; if the user issues ROLLBACK, the parent
can be allowed to continue.

Please have a look and comment. This file does not move a lot so I
don't think it will suffer from a lot of code drift.

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"I think my standards have lowered enough that now I think 'good design'
is when the page doesn't irritate the living f*ck out of me." (JWZ)