Implement dot file locking for weird platforms

[ncruces]

Implement an sqlite3_dotlk build tag compatible with unix-dotfile on all systems (same as sqlite3_flock is compatible with unix-flock).

This would allow all systems (including e.g. Plan 9) to support single process concurrency, and might allow Unixes with only broken POSIX locks (e.g. Solaris, see #168) to even support shared memory WAL.

Of course this is also of limited use, in other ways:

• only Windows/Linux/macOS/FreeBSD on amd64/arm64 support the wazero compiler
• only big endian architectures can have shared memory (Wasm is big endian)
• Windows still doesn't have mmap (Implement shared memory on Windows #75)