s3/smbd: Add a helper to fetch fs capabilities

s3/smbd: Add a helper to fetch fs capabilities

We create a new helper function to retrieve the fs capabilties via
STATVFS call. Additionally set other capabilities based on specific
parametric options. This is 99.9% taken from SMB_VFS_FS_CAPABILITIES
implementation of vfs_default in preparation to be called from any
required vfs module.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15716

Signed-off-by: Anoop C S <anoopcs@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>