libkqueue 2.5.0 no longer builds with MinGW-w64

[brechtsanders]

I was able to build libkqueue 2.4.0 for Windows with MinGW-w64, but when building libkqueue 2.5.0 there are multiple problems:

• LIST_HEAD is not defined on this platform but it is used in src/common/private.h to define kf_ready
• This platform has pthread.h, but src/windows/platform.h redefines it anyway, but it's not working:
  • pthread_mutexattr_t is used in src/common/private.h but it's not defined anywhere
  • src/windows/platform.h defines pthread_mutex_init with 2 parameters, but src/common/kqueue.c calls it with only 1

[arr2036]

It looks like the cleanups have exposed a few pre-existing issues.

Someone renamed LIST_HEAD to FIXME_LIST_HEAD in queue.h to avoid a conflict with Linux macros. That's no longer needed because we no longer include queue.h for Linux, only on platforms which actually need it.

That call to pthread_mutex_init with a single parameter has been there since at least April 2019 with a single parameter... The compatibility macro definitely takes two arguments... I'm confused as to how this used to work.

pthread_mutexattr_t isn't needed for Windows because all locks are recursive by default. I've ifdefd it out, along with the initialisation calls.