Move portal pinning from PL/pgSQL to SPI

Move portal pinning from PL/pgSQL to SPI

PL/pgSQL "pins" internally generated (unnamed) portals so that user code
cannot close them by guessing their names.  This logic is also useful in
other languages and really for any code.  So move that logic into SPI.
An unnamed portal obtained through SPI_cursor_open() and related
functions is now automatically pinned, and SPI_cursor_close()
automatically unpins a portal that is pinned.

In the core distribution, this affects PL/Perl and PL/Python, preventing
users from manually closing cursors created by spi_query and
plpy.cursor, respectively.  (PL/Tcl does not currently offer any cursor
functionality.)

Reviewed-by: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>