Problem:    Crash when using mkdir() with "R" flag in compiled function.
Solution:   Reserve a variable for deferred function calls.  Handle more than
            one argument.

Problem:    Crash when using for loop variable in closure.
Solution:   Check that the variable wasn't deleted. (issue #11094)

Problem:    Arguments in a partial not used by a :def function.
Solution:   Put the partial arguments on the stack.

Problem:    Using :defer in expression funcref not tested.
Solution:   Add a test.  Fix uncovered problems.

Problem:    Members of funccall_T are inconsistently named.
Solution:   Use the "fc_" prefix for all members.

Problem:    :defer not tested with exceptions and ":qa!".
Solution:   Test :defer works when exceptions are thrown and when ":qa!" is
            used.  Invoke the deferred calls on exit.

Problem:    Freeing the wrong string on failure.
Solution:   Adjust the argument.  Reorder the code.

Problem:    Cleaning up after writefile() is a hassle.
Solution:   Add the 'D' flag to defer deleting the written file.  Very useful
            in tests.

Problem:    Coverity warns for NULL check and unused return value.
Solution:   Remove the NULL check, it was already checked earlier.  Add (void)
            to ignore the return value.

Problem:    Cleaning up afterwards can make a function messy.
Solution:   Add the :defer command.