mount-util: clean up get_mount_flags()

mount-util: clean up get_mount_flags()

This cleans up the function in multiple ways:

- change order of parameters to follow our usualy system of putting
  return parameters last
- rename return parameter "ret" as we usually do
- don't initialize local variables we override immediately anyway
- downgrade log messages to LOG_DEBUG (since we don't log about any
  other errors here above LOG_DEBUG, as this is mostly an "API"-style
  function)
- handle that mnt_fs_get_vfs_options() may return NULL (according to
  docs)
- manually map the ST_xyz to MS_xyz flags on statvfs(), because while
  they are mostly the same, they aren't entirely the same, MS_RELATIME and
  ST_RELATIME are defined differently (sad!)