Open control FD in gofer server (and directfs gofer client) with O_PATH.

### Description

> mountpoint_s3::fuse: open failed: inode error: inode 3 (full key "synmon/test_file.txt") is not writable while being read

The issue is that gVisor's gofer client (pkg/fsimpl/gofer/) opens the file `read-only` first and then opens it for writing, but `mountpoint_s3::fuse` refuses to support that. The gofer client first caches the dentry for the file being opened. It holds a "control FD" for all the dentries it caches. This control FD is opened with O_RDONLY|O_NONBLOCK:
https://github.com/google/gvisor/blob/f67e10cf3866fe3a809c217ae0f47a9c47854ce1/pkg/sentry/fsimpl/gofer/directfs_dentry.go#L41-L50

I think we should change that logic to open regular files with O_PATH only. Directories and FIFOs can continue to be opened with O_RDONLY.

_Originally posted by @ayushr2 in https://github.com/awslabs/mountpoint-s3/issues/862#issuecomment-2093697443_