This reverts commit 547d87bfc2.

Known issues:
- To optimize dentfill() on most architectures we don't fstatat() with
  AT_SYMLINK_NOFOLLOW to get the inode number of the symlink itself.
- The current hash size serves the purpose of one filesystem. To serve
  more and have unique bits, we have to add more bits to prefix dev ID
  to the inode.  Memory consumption will be significant.

This will be reverted in next commit.

Also removed another stray cat

1. getplugs should work even if nnn is not run
2. Remove redundant ./ prefix
3. Update plugin docs

References:
1. http://man7.org/linux/man-pages/man2/posix_fadvise.2.html
2. https://www.oreilly.com/library/view/linux-system-programming/9781449341527/ch04.html

Remove extra static string.

* getpulgs now asks before overwriting

* Forgot a space

This reverts commit 407b690404.