Improve `sqlite3_dotlk` to avoid stale locks

[ncruces]

Use the locking algorithm from miquels/liblockfile (also bruceg/dotlock and nightlyone/lockfile) to avoid crashes being unrecoverable from.

Only on Unix, and only for the main database lock.

Outside Unix, keep using O_EXCL for the main database lock: it's a portable Go API for this purpose, which any Go platform should support.

For the shared memory index, keep using Mkdir to "reserve" the file name, which improves data safety, as it conflicts with the default VFSes.

[ncruces]

The libraries above make too many syscalls in the common case.

The version in 71d95bf slightly increases the chance of a crash creating a stale lock that can't be recovered from, but should cover most cases, and not leave temporary files around.

It might not support NFS as well, but using SQLite concurrently over NFS is a non-goal.