log_dir refactoring; fixes for various bugs

Move function limit test to wal/ suite, to
test without a write ahead log.

Implement cascading revoke of privilegs
on drop of space or function.

Implement a check for existing privileges
on delete from _space or _priv.

Add a test case for cascading drop of space,
which invokes revoke of privileges.

Fixes gh-289 ox.schema.user.drop() with cascade doesn't work
(a typo in schema.lua in the for loop)

Addresses gh-263 Proposal for box.cfg false/true values
and gh-277 background mode is broken (cfggeti works
correctly with boolean values now).

Fix a bug with missing initialization of
function access map (sometimes
some users could gain access to a function
simply because the respective bit in the access
map was not cleared at initialization).

Delete a duplicated stat test.

Add an error code for unknown iterator type,
ER_ITERATOR_TYPE.