configstore lock should use flock where possible

[Tim Penhey]

The configstore uses a lock file on disk (fslock) to protect against concurrent reads and writes of the environment file.

Even though this should only be held for a very short time during a read or write, a panic or signal while the fslock is held can cause it to be left behind.

On systems that support flock, we should use it, and fallback to the fslock only on Windows.