Fixup on both:
* Refactor, adding a create argument... and
* Check every hidden service directory's permissions...

Previously, we would only check the last hidden service directory.

Fixes #20529, bugfix on ticket 13942 commit 85bfad18 in 0.2.6.2-alpha.

Runs a test for each combination of create/don't create directories.

Tests #20484.

passthrough_test_setup doesn't pass through arguments if the argument
is equal to 0 or TT_SKIP. Instead, it fails or skips the test.

Assert on this, so we don't accidentally fail or skip tests.

Comment changes only

This avoids Win32 conditionals for mkdir.

check_private_dir already does this for existing directories.

It used to be rend_service_check_and_create_private_dir, which always
created the directory.

No behaviour change.

(Also, refactor the code to create a hidden service directory into a
separate funcion, so we don't have to duplicate it.)

Fixes bug 20484; bugfix on 0.2.9.3-alpha.

Previously we said we did, but didn't.

Fixes #19968; bugfix on 0.2.3.1-alpha.

The tor_fragile_assert() bug has existed here since c8a5e2d5
in tor-0.2.1.7-alpha forever, but tor_fragile_assert() was mostly a
no-op until 0.2.9.1-alpha.

Fixes bug 19869.

"ours" commit to avoid bumping version.

("ours" merge to avoid taking version bump)

This helps protect against bugs where any part of a buf_t's memory
is passed to a function that expects a NUL-terminated input.

It also closes TROVE-2016-10-001 (aka bug 20384).