OFThread.m: Move #ifdefs.

The reason is that the symbol nanosleep is available now on Win32, but
is part of the pthreads wrapper, which we don't use.
OFObject: Make _isa private.

This prevents direct access to it and enforces the usage of
object_{set,set}Class().
tests: Disable escape codes on Win32.
.gitignore: Add utils/ofzip.exe.