Fix bugs in plpgsql's handling of CALL argument lists.

exec_stmt_call() tried to extract information out of a CALL statement's
argument list without using expand_function_arguments(), apparently in
the hope of saving a few nanoseconds by not processing defaulted
arguments.  It got that quite wrong though, leading to crashes with
named arguments, as well as failure to enforce writability of the
argument for a defaulted INOUT parameter.  Fix and simplify the logic
by using expand_function_arguments() before examining the list.

Also, move the argument-examination to just after producing the CALL
command's plan, before invoking the called procedure.  This ensures
that we'll track possible changes in the procedure's argument list
correctly, and avoids a hazard of the plan cache being flushed while
the procedure executes.

Also fix assorted falsehoods and omissions in associated documentation.

Per bug #15477 from Alexey Stepanov.

Patch by me, with some help from Pavel Stehule.  Back-patch to v11.

Discussion: https://postgr.es/m/15477-86075b1d1d319e0a@postgresql.org
Discussion: https://postgr.es/m/CAFj8pRA6UsujpTs9Sdwmk-R6yQykPx46wgjj+YZ7zxm4onrDyw@mail.gmail.com
