Simplify shm_unix SharedMemoryRegistry

[niklasf]

The SharedMemoryRegistry now directly keeps a list of socket paths to unlink in the atexit handler.

Before, there were two minor issues:

• Unsynchronized access to some SharedMemory members between open() and close(AtExit), and close(AtExit) nulling data_ptr_ and search threads using it, though of course only when we were about to exit anyway.
• Moved from socket_path_ not nescessarily emptied and would then be unlinked, though I don't think any C++ implementation actually does that.

Also now just one atomic linked list away from a signal-safe cleanup_at_exit(), but one step further from a shared memory implementation that needs more general global cleanup work.

No functional change

[anematode]

The atomic linked list would be really nice... but it's not clear how to do concurrent removal.

Sopel brought up the idea of just having a bounded static vector, which could work well, but maybe incompatible with our future libfish ambitions

[niklasf]

Maybe easier if remove/add still takes a mutex, while trying to keep iteration without a mutex safe. Not sure.

[anematode]

right, well one approach I had was to keep the mutex, and atomically exchange a pointer to the std::vector (with nullptr). Then the signal handler can try to acquire it atomically, and if it's nullptr, give up. Failure would only happen with receiving a signal in the middle of construction