fix reinitialization with fuse3

fix reinitialization with fuse3

With fuse3 `fuse_get_context` returns NULL before fuse was
fully initialized, so we must not access it.

Futher, we call 'do_reload' for normal initialization as
well, so let's prevent that from re-initializing the
bindings initially and only do this on actual reloads,
otherwise we do it twice on startup.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Fixes #549