We must not (try to) modify shared values, but rather have to use our
own copies, if unixified filenames are required on Windows. To avoid
excessive string duplication, we add checks whether the filenames are
already unixified (i.e. do not contain backslashes). To improve the
performance if we need to copy strings, we use do_alloca() and friends.
Besides generally being somewhat messy, the handling of unixified
filenames is still suboptimal performance-wise, but we leave this for a
future cleanup, and focus on fixing the issue at hand for now.
We also enable opcache.protect_memory for the AppVeyor CI.
be74a00464