Paranoia: Use guaranteed_memset() to CLEAR_ON_EXIT.
SSL.File: Fixed potential hang in blocking read().

The remote close packet may already have been received,
in which case no read callback is installed anymore, and
the local backend would hang.

Also updates the documentation to mention that blocking
read and write in concurrent threads now is supported.
Testsuite: Test SSL.File in threaded blocking mode.

The main SSL.File testsuite now tests all suites in both
nonblocking callback mode and threaded blocking mode.
SSL.File: Improved robustness of closed state.

Fixes remaining bug in threaded blocking mode testsuite.
Testsuite: Keep better track of skipped SSL tests.
Testsuite: Don't test so many SSL suites in threaded mode.

Testing just the basic set of suites should be sufficient in
threaded blocking mode (as long as the corresponding nonblocking
test tests all the suites).