experimental: add darwin watchfs support

[fschade]

The current implementation of watchfs is limited to Linux hosts. While this is generally not an issue, it makes debugging watchfs-related problems on macOS (Darwin) systems difficult or impossible.

This PR introduces experimental support for watchfs on Darwin machines, gated behind the environment variable STORAGE_USERS_POSIX_WATCH_FS=true.

This implementation is not intended for production use, which is why it is hidden a build-time feature flag.

To enable it, build the binary with the Go build tag experimental_watchfs_darwin, for example:

go build -tags=experimental_watchfs_darwin

Alternatively, you can set it globally:

export GOFLAGS="-tags=experimental_watchfs_darwin"

The Darwin implementation has several known limitations:

• A hard limit of 4096 watched paths; exceeding this can cause missed events.
• No support for recursive watching, requiring us to manually watch subdirectories, which can be inefficient and resource-intensive.

For these reasons, the feature remains experimental at this time and should only be used for development or debugging purposes on macOS.

_watchfs.mp4

[micbar]

+ make lint
curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | BINDIR=/go/src/github.com/opencloud-eu/reva/toolchain sh -s v1.64.6
golangci/golangci-lint info checking GitHub for tag 'v1.64.6'
golangci/golangci-lint info found version: 1.64.6 for v1.64.6/linux/amd64
golangci/golangci-lint info installed /go/src/github.com/opencloud-eu/reva/toolchain/golangci-lint
/go/src/github.com/opencloud-eu/reva/toolchain/golangci-lint run --timeout [3](https://ci.opencloud.rocks/repos/4/pipeline/172/130#L3)00s
pkg/storage/fs/posix/tree/watch.go:19:6: func `isSubpath` is unused (unused)
func isSubpath(root, p string) bool {
     ^
pkg/storage/fs/posix/tree/watch.go:39:6: func `isPathIgnored` is unused (unused)
func isPathIgnored(tree *Tree, path string) bool {