[Patch proposal] libpq portal support

Hello everybody,

Our company was in desperate need of portals in async interface of libpq,
so we patched it.

We would be happy to upstream the changes.

The description of changes:

Two functions in libpq-fe.h:
PQsendPortalBindParams for sending a command to bind a portal to a
previously prepared statement;
PQsendPortalExecute for executing a previously bound portal with a given
number of rows.

A patch to pqParseInput3 in fe-protocol3.c to handle the `portal suspended`
message tag.

The patch is ready for review, but it lacks documentation, tests and usage
examples.

There are no functions for sending bind without params and no functions for
sync interface, but they can easily be added to the feature.

--
Thank you,
Sergei Fedorov