[subxacts] Savepoint syntax

Hackers,

Attached is the savepoints syntax patch, hopefully last try.
Essentially the same as the last patch, with the following differences:

- ROLLBACK TO changes TBLOCK state of the to-be-affected subtransactions
instead of a static var.

- A new function RollbackAndReleaseSavepoint can be used to rollback to
a savepoint without creating it again. There's no way to trigger it
from the grammar though (mainly for use in functions handlers, I
think, though we can make it available directly to the user if there
is a need for it.)

- Tab completion patch from Gaetano is included.

Please review and apply if OK.

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"The West won the world not by the superiority of its ideas or values
or religion but rather by its superiority in applying organized violence.
Westerners often forget this fact, non-Westerners never do."
(Samuel P. Huntington)